Filopodia

Functions and variants

Many cell types have filopodia. The functions of filopodia have been attributed to pathfinding of neurons, early stages of synapse formation, antigen presentation by dendritic cells of the immune system, force generation by macrophages and virus transmission. They have been associated with wound closure, dorsal closure of Drosophila embryos, chemotaxis in Dictyostelium, Delta-Notch signaling, vasculogenesis, cell adhesion, cell migration, and cancer metastasis. Specific kinds of filopodia have been given various names: microspikes, pseudopods, thin filopodia, thick filopodia, gliopodia, myopodia, invadopodia, podosomes, telopodes, tunneling nanotubes 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. Filopodia are the initial contact that human retinal pigment epithelial (RPE) cells make with elementary bodies of Chlamydia trachomatis, the bacteria that causes chlamydia.

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

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

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. Filopodia-like projections have also been linked to dendrite creation when new synapses are formed in the brain.

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.

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References

References

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