"Vesicles" is a collective term for cytoplasmic organelles that are often too small to have distinct features when imaged by light microscopy. The majority of the vesicles are membrane-bound organelles, however, also large protein complexes and cytosolic bodies can fall under this category, as they are difficult to distinguish. Examples of organelles with a vesicle annotation are the members of the endolysosomal pathway, transport vesicles including secretory granules, peroxisomes, and lipid droplets.
The biological function of an organelle is defined by its proteome (see Figure 1 for examples of proteins with a vesicular staining). In the Cell Atlas, 1998 (10%) of all human proteins have been experimentally shown to localize to vesicles (Figure 2). A Gene Ontology (GO)-based functional enrichment analysis of the vesicle proteome shows highly enriched terms for biological processes related to lipid metabolism, organization of vesicle organelles such as endosomes, vacuoles and peroxisomes, protein transport, endocytosis and exocytosis. About 63% (n=1257) of the vesicle proteins localize to one or more additional locations, with the nucleoplasm, the cytosol and the Golgi apparatus as the most common ones.
Figure 1. Examples of proteins localized to the vesicles. SNX1 is part of the retromer complex that mediates the retrograde transport of cargo proteins from endosomes to the trans-Golgi network (TGN) and is involved in endosome-to-plasma membrane transport for cargo protein recycling (detected in HeLa cells). CLTA is a structural element in clathrin-coated vesicles, which are required for the receptor-mediated endocytosis at the plasma membrane (detected in U-251 MG cells). AP2B1 is a component of the adaptor protein complex 2, which is involved in clathrin-dependent endocytosis (detected in U-2 OS cells).
Figure 2. 10% of all human protein-coding genes encode proteins localized to vesicles. Each bar is clickable and gives a search result of proteins that belong to the selected category.
The structure of vesicles
The general structure of organelles annotated as vesicles is a round membrane-enclosed lumen that is less than 1 μm in diameter. There are a few differences in the appearance of vesicles that can be seen by light microscopy and that can allow further classification, e.g. size, number or the position related to other organelles (Figure 3), but the true identity of the organelle is often only revealed by the detection of specific marker proteins in immunofluorescence images (see Table 1). Structural information about the organelles can be elucidated by electron microscopy and biochemical analyses.
Table 1. Selection of proteins suitable as markers for different vesicular organelles.
Endosomes are membrane-bound compartments that can be further sub-classified into early, recycling, and late endosomes, but there is a continuous transition between these classes. Each of them can be defined by their function, by a distinct set of proteins (e.g. members of the Rab-family of proteins (Stenmark H. 2009)), and by morphological differences. Early endosomes (EE) has a pleomorphic structure, consisting of cisternae from which two distinct subdomains emerge: large vesicular structures (300-400 nm diameter) with internal invaginations and thinner tubular extensions (60 nm diameter) (Gruenberg J. 2001). Tubular extensions give rise to recycling endosomes (RE), which retain a tubular appearance and are typically located close to the microtubule organizing center (MTOC). The large vesicular compartments of early endosomes give rise to multivesicular bodies that mediates transport to, or matures into, late endosomes. Late endosomes (LE) are again highly complex, with cisternal, tubular, and vesicular regions with numerous membrane invaginations and luminal vesicles (Griffiths G et al, 1988).
The Belgian Nobel laureate de Duve discovered the lysosome in 1955 and named it after the richness in hydrolytic enzymes (De Duve C et al, 1955). Lysosomes have a tubular morphology of about 0.1-1.2 μm in size and a characteristic acidic pH-value of 4.5-5, which is ideal for the enzymes contained in the lysosomal lumen. The membrane of lysosomes is rich in glycoproteins and consists of an unusual lipid composition that provides protection from the digestive enzymes (Schwake M et al, 2013).
The peroxisome is another organelle discovered by de Duve in 1966 (De Duve C et al, 1966), and he named them because of their involvement in peroxidase reactions. Peroxisomes originate from the ER, but they are also able to replicate themselves by division. They differ in size from 0.1-1 μm and have a dynamic structure. The shape is usually spherical, but can change and become more elongated prior to peroxisome division or in adaption to different conditions (Smith JJ et al, 2013). The elongated appearance can help to distinguish peroxisomes from other vesicles in IF.
Lipid droplets (LDs) have been known for a long time, but were believed to be a rather inert storage for lipids. The discovery of the first LD-associated protein in 1991 by Londos and coworkers (Greenberg AS et al, 1991) changed this view, and today LDs are considered organelles. LDs are formed at the ER and have a simple, yet conserved structure: a hydrophobic core containing lipids surrounded by a membrane monolayer (instead of a bilayer found in all other organelles) to which proteins are attached (Walther TC et al, 2012). The size of LDs ranges from hundreds of nanometers to the single 100 micrometer large LD that fills adipocytes. Under normal conditions, cells have no or only a few small LDs, but if those few LDs are large enough, LD-associated proteins appear in perfectly round rings and the protein location can be annotated more precisely.
Figure 3. Examples of the different types of vesicles found in the Cell Atlas. Endosomal protein RAB5C in U-2 OS cells. Lysosomal protein LAMTOR4 in U-2 OS cells. Peroxisomal protein ABCD3 in A-431 cells. LD-associated protein PLIN3 in A-431 cells. Clathrin-coated vesicle protein EPS15L1 in MCF7 cells. Vesicle-front forming protein REX1BD in U-2 OS cells.
Figure 4. 3D-view of peroxisomes in U-2 OS, visualized by immunofluorescent staining of ABCD3. The morphology of Endosomes, Lysosomes, and Peroxisomes in human induced stem cells can be seen in the Allen Cell Explorer.
The function of vesicles
Endosomes and lysosomes
Endocytosis is a process by which cells internalize extracellular solutes, ligands and proteins as well as lipids in the plasma membrane (Gruenberg J. 2001). Endocytic vesicles are typically transported to early endosomes, where efficient sorting takes place. Some lipids and membrane proteins are recycled back to the plasma membrane, either by direct routes or via recycling endosomes (Taguchi T, 2013. Some cargo enter the retrograde pathway and are delivered to the Golgi apparatus (Bonifacino JS et al, 2006). Proteins and lipids destined for degradation are instead transported to late endosomes, which then fuse with lysosomes for degradation of the material. Lysosomes contain a large spectra of enzymes, also degrading nucleic acids and carbohydrates. Materials taken up from the cytosol by autophagy are also delivered to this compartment.
Peroxisomes are multifunctional organelles that harbor a variety of enzymes and are involved in several anabolic and catabolic cellular pathways. The main function of peroxisomes is β-oxidation of long- and very long-chain fatty acids. They also contribute to the utilization and production of reactive oxygen species in the cell. In addition, peroxisomes carry out important reactions such as phospholipid biosynthesis, chemical detoxification or oxidation of purines, polyamines, and some amino acids (Antonenkov VD et al, 2010).
Nearly all cells are able to form LDs and use them as the main storage site for cellular neutral lipids. These lipids, mainly triacylglycerol and cholesterol, are utilized for the generation of energy or serve as building blocks for the synthesis of other lipids. LDs are linked to a growing number of diseases, but most prominent is their role in obesity and diabetes (Walther TC et al, 2012).
Table 2. Highly expressed single localizing proteins with a vesicular staining across different cell lines.
Gene Ontology (GO)-based enrichment analysis of genes encoding proteins that localize mainly to vesicles reveals several functions associated with the group of organelles comprised of vesicles. The highly enriched terms for the GO domain Biological Process are related mainly to lipid metabolism, which is connected to processes in peroxisomes, and processes related to the function of endosomes (Figure 5a). For the GO domain Molecular Function, vesicle proteins are enriched for sterol transport and receptor related actions (Figure 5b).
Figure 5a Gene Ontology-based enrichment analysis for the vesicles proteome showing the significantly enriched terms for the GO domain Biological Process. Each bar is clickable and gives a search result of proteins that belong to the selected category.
Figure 5b Gene Ontology-based enrichment analysis for the vesicles proteome showing the significantly enriched terms for the GO domain Molecular Function. Each bar is clickable and gives a search result of proteins that belong to the selected category.
Vesicles proteins with multiple locations
Approximately 63% (n=1257) of the vesicle proteins detected in the Cell Atlas also localize to other compartments in the cell. The network plot (Figure 6) shows that the most overrepresented locations with vesicles are the nucleoplasm, the ER, and the Golgi apparatus. Given the function of vesicles, these multiple locations is in agreement with their role in the secretory pathway. Examples of multilocalizing proteins within the proteome of vesicles can be seen in Figure 7.
Figure 6. Interactive network plot of vesicle-associated proteins with multiple localizations. The numbers in the connecting nodes show the proteins that are localized to vesicles and to one or more additional locations. Only connecting nodes containing more than one protein and at least 0.5% of proteins in the vesicle proteome are shown. The circle sizes are related to the number of proteins. The cyan colored nodes show combinations that are significantly overrepresented, while magenta colored nodes show combinations that are significantly underrepresented as compared to the probability of observing that combination based on the frequency of each annotation and a hypergeometric test (p≤0.05). Note that this calculation is only done for proteins with dual localizations. Each node is clickable and results in a list of all proteins that are found in the connected organelles.
Figure 7. Examples of multilocalizing proteins in the vesicle proteome. The examples show common or overrepresented combinations for multilocalizing proteins in the proteome. VTI1B promotes the fusion of vesicles with the target membrane at the Golgi apparatus (detected in U-2 OS cells). GPRC5A, detected at the plasma membrane and vesicles, is an orphan receptor, which might be involved in the interaction between retinoic acid and G-protein signaling pathways (detected in U-2 OS cells). EPN3 co-localizes with clathrin-coated vesicles and shuttles into the nucleus (detected in HaCaT cells).
Expression levels of vesicles proteins in tissue
Transcriptome analysis and classification of genes into tissue distribution categories (Figure 8) shows that a larger portion of the vesicle proteins are detected in some or in many tissues, while a smaller portion are detcted in all tissues, compared to all genes presented in the Cell Atlas. This may reflect a variety of tissue-specific functions involving vesicle proteins, particularly in the transport of secretory proteins and other biomolecules to the outside of the cell.
Figure 8. Bar plot showing the percentage of genes in different tissue distribution categories for vesicle-associated protein-coding genes, compared to all genes in the Cell Atlas. Asterisk marks a statistically significant deviation (p≤0.05) in the number of genes in a category based on a binomial statistical test. Each bar is clickable and gives a search result of proteins that belong to the selected category.
Relevant links and publications
Thul PJ et al, 2017. A subcellular map of the human proteome. Science.