The Biogenesis of Exosomes: How They Form and Function in the Body

Exosomes are tiny extracellular vesicles, approximately 30–150 nm in diameter, that play a vital role in intercellular communication by transporting proteins, lipids, and nucleic acids between cells. Understanding their biogenesis, the process by which they form and are released, is essential for appreciating their functions in health, disease, and therapeutic applications.

Step 1: Initiation within the Endosomal System

Exosome biogenesis begins inside the cell within the endosomal pathway. The plasma membrane invaginates to form early endosomes, which mature into late endosomes or multivesicular bodies (MVBs). During this maturation, the endosomal membrane undergoes inward budding, creating numerous small vesicles inside the MVBs called intraluminal vesicles (ILVs). These ILVs are the precursors of exosomes  1-2-5.

Step 2: Cargo Sorting and ILV Formation

The sorting of molecular cargo—such as proteins, RNAs, and lipids—into ILVs is a tightly regulated process involving multiple mechanisms:

• ESCRT-Dependent Pathway: The Endosomal Sorting Complex Required for Transport (ESCRT) machinery is central to ILV formation. It consists of four protein complexes (ESCRT-0, -I, -II, and -III) and associated proteins like VPS4. ESCRT-0, -I, and -II recognize and sequester ubiquitinated cargo, while ESCRT-III facilitates membrane deformation and scission to release ILVs into the MVB lumen 1-2.
• ESCRT-Independent Pathways: Alternative mechanisms involving lipid rafts, tetraspanins, and ceramide also contribute to cargo sorting and ILV formation, adding to the heterogeneity of exosomes 5.

The specific cargo loaded into ILVs determines the functional properties of the resulting exosomes.

Step 3: MVB Fate—Degradation or Exosome Release

Once formed, MVBs have two main fates:

• Fusion with Lysosomes: Leading to degradation of ILVs and their cargo.
• Fusion with the Plasma Membrane: Resulting in the release of ILVs into the extracellular space as exosomes 357.

The decision between these pathways is regulated by cellular signals and molecular machinery, including the cytoskeleton and Rab GTPases that mediate MVB transport and docking at the plasma membrane.

Step 4: Exosome Secretion and Uptake

Upon fusion of MVBs with the plasma membrane, exosomes are secreted into the extracellular environment. They can act locally or travel through body fluids to distant sites, mediating autocrine, paracrine, or endocrine signaling 6.

Target cells uptake exosomes via several mechanisms:

• Direct fusion with the plasma membrane.
• Endocytosis (including phagocytosis or micropinocytosis).
• Receptor-ligand interactions facilitating internalization 6.

Regulation and Functional Implications

Exosome biogenesis is highly dynamic and influenced by cell type, microenvironment, and physiological or pathological conditions. For example, cancer cells can manipulate exosome production and cargo to promote tumor progression 7-8.

Moreover, the molecular cargo within exosomes can regulate their own biogenesis, creating feedback loops that fine-tune intercellular communication5.

Summary

Exosome biogenesis is a multistep, tightly controlled process beginning with endocytosis and ILV formation within MVBs, followed by MVB trafficking and fusion with the plasma membrane to release exosomes. The ESCRT machinery plays a pivotal role, but alternative pathways also contribute. Released exosomes serve as critical messengers, transferring diverse molecular cargo to recipient cells and influencing numerous biological processes.

References

1. Current knowledge on exosome biogenesis and release - PMC (2017)
2. Regulation of cargo selection in exosome biogenesis - Nature (2024)
3. The biogenesis and secretion of exosomes - ScienceDirect
4. The Machinery of Exosomes: Biogenesis, Release, and Uptake - MDPI (2023)
5. The role of exosomal molecular cargo in exosome biogenesis - Frontiers in Immunology (2024)
6. Biogenesis of exosomes - Everzom (2022)
7. Exosome biogenesis: machinery, regulation, and therapeutic implications in cancer - PMC (2022)
8. Exosome biogenesis – machinery, regulation, and therapeutic implications in cancer - Exosome RNA