Endosomal

Endosomal refers to the endosome, a membrane-bound compartment inside eukaryotic cells that forms as part of the endocytic pathway. Endosomes originate from the plasma membrane during uptake and then serve as sorting stations for internalized material. They determine whether cargo is recycled back to the surface, sent to the trans-Golgi network, or delivered to lysosomes for degradation.

Endosomes are organized by maturation state. Early endosomes, marked by Rab5 and phosphatidylinositol 3-phosphate, act as primary sorting hubs for internalized cargo. They can recycle receptors to the plasma membrane via recycling endosomes, or progress to late endosomes. Late endosomes, which express Rab7 and lysosome-associated membrane proteins, fuse with lysosomes or develop into multivesicular bodies that contain intraluminal vesicles. Recycling endosomes, often Rab11-positive, retrieve receptors and other cargo for reuse.

The maturation process involves coordinated changes in small GTPases and effector proteins, sometimes referred to as Rab conversion, along with ESCRT machinery that sorts ubiquitinated cargo into intraluminal vesicles within multivesicular bodies. Lipid signaling, particularly phosphoinositides, and SNARE-mediated membrane fusion also regulate endosomal trafficking and fusion events with lysosomes or other compartments.

In addition to degradation, endosomes influence signaling by modulating receptor availability and sustaining signaling from internalized receptors. They also participate in antigen processing and presentation in immune cells.

Dysfunction of endosomal-lysosomal trafficking is linked to diseases such as neurodegenerative disorders and lysosomal storage diseases. Pathogens may exploit endosomes to gain entry, and endosomal pathways are areas of interest for drug delivery and gene therapy, where achieving endosomal escape remains a key challenge.