Breaking cellular boundaries: molecular mechanisms of tunneling nanotube formation and fusion.

Tunneling nanotubes (TNTs) are thin, actin-based membrane bridges that establish direct cytoplasmic continuity between distant cells, enabling the transfer of diverse cargoes ranging from ions and proteins to organelles such as mitochondria. Since their discovery in 2004, TNTs have been identified in numerous cell types and linked to an expanding range of physiological and pathological functions. Yet, their molecular identity and mechanisms of formation remain elusive. The most defining and least understood step in TNT biogenesis is membrane fusion, the process by which TNTs achieve open-ended continuity between cells, and this represents a critical frontier in the field. This review integrates recent advances in TNT biology, emphasizing the interplay between actin cytoskeletal dynamics, plasma membrane composition, and cell adhesion during TNT formation. It also draws mechanistic parallels with established models of membrane fusion, highlighting fundamental principles and shared regulators across fusion systems, many of which have been implicated in TNT functionality. By combining molecular, biophysical, and imaging perspectives, this review proposes a conceptual framework for TNT formation and fusion, identifies major methodological gaps, and outlines future directions to unravel the mechanisms that underlie intercellular cytoplasmic continuity.