The Hemifusome: A Paradigm Shift in Cellular Cargo Sorting and a New Frontier in Disease Research

The endolysosomal system is crucial for cellular homeostasis. Historically, the ESCRT machinery was considered the sole mechanism responsible for sorting cargo and generating multivesicular bodies (MVBs). However, the discovery of the hemifusome a novel organelle with a distinct 'snowman-like' architecture challenges this traditional view. Composed of two vesicles connected by a stable hemifusion diaphragm and organized by a proteolipid nanodroplet, the hemifusome functions as a dynamic platform for cargo sorting independent of the ESCRT pathway. This review synthesizes current knowledge regarding the hemifusome's structure and function, presenting it as a paradigm shift in intracellular trafficking. Furthermore, we propose a new pathological framework termed "hemifusome-opathy." We postulate that hemifusome dysfunction is a primary driver of major human diseases, specifically contributing to the toxic protein accumulation seen in neurodegenerative conditions like Alzheimer's and Parkinson's disease. By outlining a roadmap for future research, we identify the hemifusome as a critical new frontier in cell biology with significant potential for developing novel diagnostic and therapeutic strategies.