Hemifusomes and proteolipid nanodroplets: a critical synthesis of an emerging model for endosomal membrane remodeling

Vesicular membrane trafficking is central to eukaryotic homeostasis, governing receptor downregulation, lysosomal degradation, and intercellular communication via exosomes. These processes are classically explained by SNARE-mediated fusion and ESCRT-dependent membrane remodeling, which together define current models of vesicle biogenesis and multivesicular body (MVB) formation. Recent high-resolution cryo-electron tomography of intact mammalian cells has identified a structurally distinct membrane-associated assembly termed the hemifusome. Hemifusomes consist of two heterotypic vesicles connected by a persistent hemifusion diaphragm (~ 160 nm in diameter) and feature a ~ 42 nm proteolipid nanodroplet (PND) localized at the diaphragm rim. This architecture expands current views of hemifusion intermediates and raises the possibility of an alternative mode of endosomal membrane organization distinct from canonical ESCRT-mediated processes. However, the molecular composition, biogenesis, and regulatory mechanisms of this system remain undefined. Here, we provide the first integrated synthesis of hemifusome and PND biology, consolidating structural, biophysical, and mechanistic observations within the context of endosomal trafficking. We compare this emerging framework with ESCRT-mediated intraluminal vesicle formation, highlighting both shared features and key mechanistic gaps. We further examine preliminary and largely correlative links to neurodegenerative and lysosomal storage disorders, while emphasizing the absence of direct causal evidence. Given that this field currently rests on a single primary research report, we deliberately separate this evidence-based structural synthesis from more speculative functional, disease-related, and translational extensions, which are presented as explicit future perspectives rather than established conclusions. Finally, we outline unresolved questions, including PND molecular identity, determinants of hemifusion diaphragm organization and stability, and the temporal sequence of hemifusome assembly and remodeling. Together, these findings position hemifusomes as a potentially distinct structural state within the endosomal network, warranting systematic molecular and functional investigation.