filopodun

Filopodun are elongated, thin cellular extensions that have been identified in a limited number of eukaryotic cell types. They resemble classic filopodia in appearance, projecting from the plasma membrane at the leading edge of migrating cells, yet they possess distinct ultrastructural and functional characteristics.

Microscopic examination reveals that filopodun are typically 5 to 10 micrometres in length and only 50 to 150 nanometres in width, rendering them visibly thinner than conventional filopodia. Unlike filopodia that largely rely on bundled actin filaments for traction, filopodun incorporate both actin and microtubule components. The presence of microtubules confers additional rigidity and permits the transport of vesicles and signaling complexes along the shaft, facilitating rapid communication with the extracellular matrix.

Biochemically, filopodun express a unique set of membrane proteins, notably a distinct isoform of the integrin β1 subunit and a high concentration of protocadherin-like molecules. These differences suggest specialized roles in cell–cell adhesion and mechanosensing that are not shared by conventional filopodia. Functional studies in cultured neuronal and immune cells indicate that filopodun are crucial for maintaining dendritic spine stability and for guiding immune synapse formation during antigen presentation.

The development of filopodun appears to be regulated by a distinct signalling cascade involving the small GTPase RhoC and the adaptor protein IQGAP1. Activation of this pathway promotes the assembly of the mixed actin–microtubule cytoskeleton and upregulates the expression of filopodun-specific membrane proteins.

Despite growing evidence for their physiological importance, the precise biological functions of filopodun remain incompletely understood, and further investigations are needed to ascertain their roles in development, wound healing, and disease pathogenesis.