Actin-based protrusions are at the base of many fundamental cellular processes, such as cell adhesion, migration and intercellular communication. In recent decades, the discovery of new types of actin-based protrusions with unique functions has enriched our comprehension of cellular processes. However, as the repertoire of protrusions continues to expand, the rationale behind the classification of newly identified and previously known structures becomes unclear. Although current nomenclature allows good categorization of protrusions based on their functions, it struggles to distinguish them when it comes to structure, composition or formation mechanisms. In this Cell Science at a Glance article, we discuss the different types of actin-based protrusions, focusing on filopodia, cytonemes and tunneling nanotubes, to help better distinguish and categorize them based on their structural and functional differences and similarities.

Tunnelling nanotubes (TNTs) connect distant cells and mediate cargo transfer for intercellular communication in physiological and pathological contexts. How cells generate these actin‐mediated protrusions to span lengths beyond those attainable by canonical filopodia remains unknown. Through a combination of micropatterning, microscopy, and optical tweezer‐based approaches, we demonstrate that TNTs formed through the outward extension of actin achieve distances greater than the mean length of filopodia and that branched Arp2/3‐dependent pathways attenuate the extent to which actin polymerizes in nanotubes, thus limiting their occurrence. Proteomic analysis using epidermal growth factor receptor kinase substrate 8 (Eps8) as a positive effector of TNTs showed that, upon Arp2/3 inhibition, proteins enhancing filament turnover and depolymerization were reduced and Eps8 instead exhibited heightened interactions with the inverted Bin/Amphiphysin/Rvs (I‐BAR) domain protein IRSp53 that provides a direct connection with linear actin polymerases. Our data reveals how common protrusion players (Eps8 and IRSp53) form tunnelling nanotubes, and that when competing pathways overutilizing such proteins and monomeric actin in Arp2/3 networks are inhibited, processes promoting linear actin growth dominate to favour tunnelling nanotube formation.

Over the years, the influence of secretory mechanisms on intercellular communication has been extensively studied. In the central nervous system (CNS), both trans-synaptic (neurotransmitter-based) and long-distance (extracellular vesicles-based) communications regulate activities and homeostasis. In less than a couple of decades, however, there has been a major paradigm shift in our understanding of intercellular communication. Increasing evidence suggests that besides secretory mechanisms (via extracellular vesicles), several cells are capable of establishing long-distance communication routes referred to as Tunneling Nanotubes (TNTs). TNTs are membranous bridges classically supported by F-Actin filaments, allowing for the exchange of different types of intracellular components between the connected cells, ranging from ions and organelles to pathogens and toxic protein aggregates. The roles of TNTs in pathological spreading of several neurodegenerative conditions such as Prion diseases, Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD) have been well established. However, the fragile nature of these structures and lack of specific biomarkers raised some skepticism regarding their existence. In this review, we will first place TNTs within the spectrum of intercellular communication mechanisms before discussing their known and hypothesized biological relevance in vitro and in vivo in physiological and neurodegenerative contexts. Finally, we discuss the challenges and promising prospects in the field of TNT studies.