Endosomes

Endosomes are membrane-bound compartments within cells that originate from endocytosis and from the trans-Golgi network in some pathways. They function as central sorting stations for internalized material, regulating receptor signaling, nutrient uptake, and membrane composition. Endosomes receive cargo from the plasma membrane and determine whether it should be recycled back to the surface or delivered to lysosomes for degradation.

Endosomes are commonly categorized as early endosomes, recycling endosomes, and late endosomes. Early endosomes, marked by Rab5, receive internalized material and maintain a mildly acidic lumen. From here, cargo and receptors can be recycled back to the plasma membrane via recycling endosomes, often Rab11-positive, or mature into late endosomes for degradation. Late endosomes, which are more acidic, frequently coincide with multivesicular bodies that contain intralumenal vesicles formed by the ESCRT machinery to sequester ubiquitinated cargo for degradation. Late endosomes fuse with lysosomes to deliver their cargo to hydrolases for breakdown.

The trafficking between endosomal compartments is orchestrated by small GTPases and tethering factors, with Rab family proteins playing a central role. Sorting of cargo involves ESCRT complexes that recognize ubiquitinated proteins, while the retromer complex can retrieve certain receptors from endosomes back to the Golgi or plasma membrane. Endosomes also participate in antigen processing and presentation in immune cells and influence signaling by regulating receptor availability on the cell surface.

Disruptions in endosomal trafficking are associated with various diseases, including neurodegenerative disorders and immune dysfunction.