Multivesicular Bodies
Multivesicular bodies (MVBs) are late endosomes with single-layered membranes and a diameter ranging from 200 to 1000 nm. They contain multiple small vesicles in their lumen. MVBs were originally observed in the nervous system in the 1950s, and their defining gold standard is the presence of a large vesicle containing multiple intraluminal vesicles (ILVs) under electron microscopy. As an important cellular organelle, MVBs are primarily responsible for delivering cargo molecules from the endocytic pathway to lysosomes for degradation and recycling, thereby regulating biological processes such as nutrient uptake, immunity, and signal transduction.
Additionally, MVBs can also fuse with the plasma membrane to release ILVs, serving as extracellular vesicles for intercellular communication. Under pathological conditions, tumor-derived exosomes released by MVBs can alter the tumor microenvironment, promote angiogenesis, and modulate the host immune system to facilitate tumor progression. Therefore, the study of the mechanisms underlying MVB formation is of great physiological and clinical significance as it is a key delivery pathway to lysosomal degradation and the origin of exosome generation.
Mechanisms of Multivesicular Bodies Formation
MVB widely exists in eukaryotic cells from yeast to higher mammals, and its morphology, distribution, contents, and intracellular destination are highly heterogeneous and are associated with specific physiological functions. There are diversifications in cells mechanism mediating the generation of distinct MVB taxa. In recent years, researchers have discovered a number of factors that regulate MVB formation. According to their effector proteins and mechanism of action, the mechanism of MVB formation can be divided into three categories: ESCRT complex-dependent mechanism, specific lipid molecule-driven mechanisms, and mechanisms driven by tetraspanins.
1. SCRT-dependent MVB generation mechanism
The endosomal sorting complexes required for transport (ESCRT) play a crucial role in the formation of multivesicular bodies. The classical ESCRT pathway consists of five different complexes, ESCRT-0, -I, -II, -III, and the associated AAA-ATPase complex VPS4. In the formation of degradative MVBs, ESCRT-0 is recruited to the endosomal membrane by phosphatidylinositol 3-phosphate (PI3P) and ubiquitinated receptor proteins. ESCRT-0 then interacts with the meshwork proteins to aggregate on the membrane, forming microdomains and concentrating cargo proteins at specific locations. ESCRT-0 recruits ESCRT-I, which subsequently delivers ubiquitinated cargo proteins to ESCRT-I. ESCRT-I then recruits ESCRT-II, forming a supercomplex that induces membrane deformation of the endosomal membrane and concentrates sorted cargo proteins at the budding sites. ESCRT-II continues to recruit ESCRT-III, promoting the polymerization of ESCRT-III components in the neck region of the invaginated vesicle, further constricting the neck region of the vesicle and inducing cargo protein deubiquitination. Finally, the VPS4 complex disassembles the ESCRT-III complex, facilitating vesicle scission from the membrane and recycling of ESCRT-III components.
Fig.1 A model for the organization of the ESCRT system (Hurley J.H. 2008).
2. Mechanism of MVB generation driven by specific lipid molecules
In addition to ESCRT components and their associated proteins, MVB formation can also be mediated by specific lipid molecules independent of the ESCRT pathway. Lipids found to be involved in the regulation of MVB formation mainly include ceramide, phosphatidic acid, and sphingosine-1-phosphate. Different from the classic ESCRT pathway research, the specific mechanism of action of these lipids is still not very clear. According to the properties of the identified lipid regulatory factors and related research results, the researchers proposed the following three hypotheses:
(1) Ceramide can promote the formation and further expansion of sphingomyelin-rich lipid raft microdomains on the endosomal membrane, thereby inducing the budding growth of the endosomal membrane.
(2) Lipids such as ceramide and phosphatidic acid are similar in structure to cones, they have smaller hydrophilic heads and larger hydrophobic tails, and thus induce spontaneous negative curvature (i.e., the membrane is concave inward) when embedded in endosomal membranes, thereby promoting ILV formation.
(3) Sphingosine-1-phosphate (S1P) can act as a lipid signal to activate the downstream S1P receptor. Activated S1P receptor regulates MVB formation by activating the small G proteins CDC42 and Rac1 that regulate microfilament polymerization, which promotes microfilament polymerization on the membrane of the endosomes to facilitate cargo sorting to the ILV.
3. Mechanism of MVB generation driven by tetraspanins
In addition, recent studies have also identified multiple members of the four-transmembrane family of proteins (CD63, CD9, CD82, CD81, etc.) involved in regulating the production of secreted MVB and exosomes, but the specific regulatory mechanism is still not very clear. According to the characteristics and research results of this family of proteins, the researchers put forward the hypothesis that the tetraspanin family proteins interact with more proteins, and this type of protein also easy to forms multimeric complexes, so it tends to include Clusters clustered on the endosome membrane to form the tetraspanin-enriched microdomain, which can effectively promote the sorting and enrichment of other cargo proteins on the endosome membrane and the formation of ILV.
Although there are many studies on MVB generation, and many regulatory factors and mechanisms mediating ILV formation have been revealed, there are still many unsolved problems in the field of MVB.
References
- Hurley J.H. ESCRT complexes and the biogenesis of multivesicular bodies. Current Opinion in Cell Biology. 2008, 20 (1): 4-11.
- Jarsch I.K.; et al. Membrane curvature in cell biology: An integration of molecular mechanisms. Journal of Cell Biology. 2016, 214 (4): 375-87.
- Rana S.; Zöller M. Exosome target cell selection and the importance of exosomal tetraspanins: a hypothesis. Biochemical Society Transactions. 2011, 39 (2): 559-562.
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