Exosome - intercellular signal carrier with future potential
Can Provide New Diagnostic and Therapeutic Possibilities
Gunnar Ronquist, Professor Emeritus, Department of Clinical Chemistry, Department of Medical Sciences, Uppsala University
Jan Lötvall, Professor and Chief Physician, Krefting Research Center, University of Gothenburg
Susanne Gabrielsson, Associate Professor, Unit for Translational Immunology, Department of Medicine, Karolinska University Hospital, Solna; Karolinska Institutet, Stockholm
Lucia Mincheva-Nilsson, Professor and Chief Physician, Department of Clinical Immunology, Department of Clinical Microbiology, Umeå University
Joar Svanvik, Professor Emeritus, Transplantation Center, Sahlgrenska University Hospital, Gothenburg
Esbjörn Telemo, Professor and Chief Physician, Department of Rheumatology and Inflammation Research, University of Gothenburg
Anders Waldenström, Professor Emeritus, Department of Public Health and Clinical Medicine, Umeå University; Cardiology, Heart Center, Norrland University Hospital, Umeå
Extracellular Vesicles: Important Messengers Between Cells
Virtually all cells in the human body have the ability to release small membrane-enclosed structures known as extracellular vesicles (EVs). These vesicles play an important role in communication between cells by transporting biological material from one cell to another. They can be considered natural messengers that allow cells to exchange information and influence the activities of surrounding or distant cells.
Extracellular vesicles are generally divided into three main subgroups according to their characteristics and mechanisms of formation: exosomes, microvesicles and apoptotic bodies. Their sizes vary considerably, ranging from approximately 30 nanometres to more than 1,000 nanometres. Although they share certain structural characteristics, these vesicles differ in their formation processes, composition and biological functions.
Among these different types, exosomes are particularly small structures. They can be produced by virtually every type of cell and have been identified in several biological fluids throughout the body. Their presence in these fluids, combined with their ability to transport biologically active molecules, has attracted considerable scientific interest.
Exosomes are especially important because they participate in intercellular communication. By transferring selected biological components between cells, they can influence cellular processes and modify the behaviour of recipient cells. This communication mechanism has become an important subject of investigation in cell biology, immunology and medical research.
Exosomes can communicate with target cells through different mechanisms. One possibility is the delivery of functional proteins and/or RNA molecules into the cytoplasm of recipient cells. Once these molecules enter the target cell, they may influence its biological activities. Another mechanism involves direct interactions between molecules located on the exosomal membrane and receptors present on the surface of another cell. These interactions can initiate biological signals without necessarily requiring the internalisation of the entire vesicle.
Potential Clinical Applications in Diagnosis and Therapy
The biological properties of exosomes have opened up interesting possibilities for clinical applications, particularly in the fields of disease diagnosis and treatment. Research is exploring how these vesicles could be used as tools for detecting diseases, transporting therapeutic molecules and improving the delivery of biological treatments.
One promising approach involves loading endogenous exosomes with cancer-specific peptides. These modified vesicles may then be investigated as potential therapeutic tools in human cancer treatment trials. Their natural origin and ability to interact with other cells make them interesting candidates for transporting biologically active substances.
Furthermore, the proteins and RNA molecules contained in exosomes may provide valuable information about the cells from which they originate. Since exosomes can reflect the biological condition of their parent cells, analysing their molecular content could help identify changes associated with diseases, including different types of cancer. This makes them potential sources of biomarkers for future diagnostic applications.
Another important possibility is their use as natural carriers of RNA molecules. Exosomes could also serve as biological containers for gene therapy vectors, offering a potential method for delivering genetic material into specific target cells. These applications could contribute to the development of new therapeutic strategies.
The Exosomes Are Formed in the Endosomal Machinery of the Cell
To understand the biological functions of exosomes, it is important to examine how they are produced inside cells. Their formation is closely associated with the endosomal system, which is involved in the internalisation, sorting and transport of molecules within the cell.
Endocytosis is the process through which cells take up molecules from their surrounding environment. During this process, portions of the cell membrane are internalised, allowing membrane-associated molecules, including lipids and proteins, as well as extracellular material, to enter the cell. These components are subsequently transported through intracellular compartments, where they can be sorted, recycled or directed towards other cellular destinations.
The formation of exosomes is part of this complex intracellular trafficking system. Proteins that are eventually secreted through exosomes can reach the endosomal compartments through different pathways. Some are first located at the cell membrane and are then internalised through endocytosis. Others may be transported through recycling pathways involving specialised membrane regions known as lipid rafts. Proteins newly synthesised within the cell can also be transported from the Golgi apparatus towards the endosomal system.
The membrane surrounding exosomes consists of a phospholipid bilayer, similar to the membrane of the cell from which they originate. This membrane contains several types of lipids and proteins that contribute to the structure and biological properties of the vesicles.
A particularly important component of this membrane organisation is the presence of lipid rafts. These are specialised membrane domains enriched in sphingomyelin and cholesterol. They can selectively attract and concentrate certain proteins involved in cellular communication and signal transduction. Consequently, lipid rafts may contribute to the selective incorporation of specific proteins into the developing exosomes.
The initial stages of exosome formation involve the inward invagination of these membrane regions. This process contributes to the formation of early endosomes, which subsequently develop into more complex endosomal structures.
As the endosomal compartment matures, additional inward invaginations of its membrane occur. These invaginations generate small vesicles inside the endosome, known as intraluminal vesicles. When an endosome contains numerous intraluminal vesicles, it is referred to as a multivesicular body (MVB).
The formation of these internal vesicles is an important characteristic of exosome biogenesis. Initially, the plasma membrane invaginates inwards, allowing material from the cell surface to enter the cell. Subsequently, the membrane of the endosome invaginates again, producing vesicles within the endosomal compartment. This double-invagination process results in the preservation of the normal membrane orientation of the original cell membrane components.
This mechanism is particularly important because it allows proteins incorporated into exosomes to retain their orientation within the membrane. As a result, their three-dimensional structure and biological properties can be preserved. This is essential for maintaining their ability to interact with receptors and participate in biological signalling processes.
Following their formation, multivesicular bodies can fuse with the plasma membrane. This fusion event releases the intraluminal vesicles into the extracellular environment. Once released outside the cell, these vesicles are referred to as exosomes.
Exosomes are generally spherical structures with a diameter ranging from approximately 30 to 100 nanometres. Their membranes are particularly enriched in sphingomyelin, cholesterol and proteins belonging to the tetraspanin family. These membrane components contribute to their structural organisation and interactions with other cells.
Proteins originally associated with lipid rafts can also be found on the surface of exosomes. Some of these proteins function as ligands that recognise and bind to receptors present on the membranes of other cells. Through these interactions, exosomes can attach to specific target cells and potentially deliver their molecular content.
The composition of exosomes is not random. They contain specific proteins and RNA molecules, and may also carry fragments of DNA. Their molecular content reflects, to a certain extent, the characteristics of the cell from which they originate. Consequently, changes in the physiological condition of the parent cell can influence the composition of the released exosomes.
For example, cellular activation or stress may modify the types and quantities of molecules incorporated into exosomes. This means that exosomes released under different biological conditions may carry different molecular information and produce different effects on recipient cells.
Exosomes' Ability to Interact With Other Cells
Exosomes are involved in a wide range of biological processes, and their functions vary depending on the cell types involved and the molecular content of the vesicles. Their ability to transport proteins, RNA and other biological components makes them important participants in communication between different tissues and cellular systems.
Research has demonstrated that exosomes can influence both immunostimulatory and immunosuppressive processes. Depending on their origin and composition, they may stimulate immune responses or contribute to mechanisms that reduce immune activity. They can also participate in the regulation of the coagulation cascade, influence the development of metastases and contribute to the regulation of cardiac functions.
The term exosome was introduced in 1987. However, the observation of extracellular vesicles dates back to earlier investigations. Approximately ten years before the introduction of this term, researchers had already identified vesicles in prostate secretions. These structures, known as prostasomes, were found to interact with sperm cells, probably through adhesion molecules located on their surfaces.
Prostasomes belong to the broader family of extracellular vesicles and illustrate how these structures can influence the biological properties of other cells. One of their proposed functions is to protect sperm cells within the female genital tract. In particular, prostasomes contain CD59, a protein that can interfere with the complement system and help protect sperm cells from complement-mediated damage.
Interestingly, this protective mechanism may also be exploited by malignant cells. When CD59 is transferred from extracellular vesicles to cancer cells, it can provide these cells with protection against immune mechanisms that would normally contribute to their elimination. This illustrates how extracellular vesicles can influence interactions between cells and the surrounding immune environment.
Exosomes are also involved in immune regulation within the intestinal tract. The intestinal epithelium releases vesicles that can contribute to the development of immune tolerance. These vesicles, sometimes referred to as tolerosomes, are associated with mechanisms that help prevent allergic sensitisation to food components.
Oral tolerance is an important biological process through which the immune system develops a controlled response to substances introduced through the digestive tract. By participating in this process, intestinal exosomes may contribute to maintaining an appropriate balance between immune protection and tolerance towards dietary components.
The placenta is another important source of extracellular vesicles. During normal pregnancy, the syncytiotrophoblast layer produces and releases large quantities of exosomes. These vesicles have immunosuppressive properties and contain proteins capable of downregulating certain cytotoxic functions of maternal immune cells.
The continuous secretion of placental exosomes creates a concentration gradient of fetal-derived vesicles in the maternal circulation. This distribution may contribute to the establishment of a protective environment around the fetus and placenta. By influencing maternal immune responses, these vesicles help limit immune mechanisms that could otherwise attack fetal or placental tissues. Their activity is therefore considered important for maintaining the immunological conditions required for pregnancy to proceed successfully.
Another significant characteristic of exosomes is their ability to transport functional genetic material. Studies have demonstrated that they can contain messenger RNA (mRNA) and microRNA molecules that can be transferred from one cell to another. Once delivered to recipient cells, these RNA molecules may influence gene expression and modify cellular functions.
Interestingly, the RNA content of exosomes does not necessarily correspond directly to the overall RNA composition of their parent cells. This observation suggests that RNA molecules may be selectively incorporated into exosomes rather than being transferred randomly. Such selective loading could help explain why exosomes carry specific biological messages.
The molecular composition of exosomes can also change when their parent cells experience stress. Changes in their RNA content may consequently influence their biological activity and the effects they produce in recipient cells. This highlights the importance of the physiological condition of the originating cell in determining the functions of extracellular vesicles.
Human breast milk provides another example of the potential immunological functions of exosomes. Vesicles isolated from breast milk have been reported to induce regulatory T cells expressing Foxp3. These cells are involved in controlling immune responses and maintaining immune tolerance. Such observations suggest that breast milk exosomes may contribute to the development of immune regulation in breastfeeding infants.
In addition to their protein content, breast milk exosomes also contain RNA molecules, further supporting their potential role as biological carriers. Their molecular components may participate in communication between maternal and infant cells.
Extracellular vesicles are also found in bronchoalveolar lavage fluid, which is collected from the respiratory tract. In patients with inflammatory conditions such as sarcoidosis and asthma, these samples can contain pro-inflammatory exosomes. Their presence suggests that extracellular vesicles may participate in inflammatory processes within the lungs and contribute to communication between cells involved in respiratory immune responses.
The cardiovascular system provides another example of extracellular vesicle-mediated communication. Cardiomyocytes, the muscle cells responsible for heart contraction, can release both exosomes and microvesicles containing DNA. Experimental observations have shown that this DNA can be transferred into recipient cells, including fibroblasts, where it may remain functional.
These findings indicate that extracellular vesicles can transport genetic material between different cell types. Moreover, the biological environment surrounding the parent cell can influence the quality and molecular composition of the released vesicles. As a result, the effects observed in recipient cells following the transfer of exosomal material may depend on the physiological condition of the originating cardiomyocytes.
The Future of Exosome Research Is Bright and Exciting
The increasing interest in extracellular vesicles has opened several important research directions. Their involvement in intercellular communication, immune regulation and the transport of biological molecules makes them promising subjects for both fundamental and clinical investigations.
Although many functions have already been identified, a considerable amount of research is still needed to understand the mechanisms responsible for their formation, molecular composition and biological activity.
Fundamental Research in Cell Biology and Immunology
One of the main priorities is to improve our understanding of the basic mechanisms governing exosome biology. Researchers still need to clarify how specific proteins and RNA molecules are selected and incorporated into developing vesicles, how exosomes are released from cells and how they recognise their target cells.
Another important question concerns the mechanisms through which exosomes deliver their molecular content to recipient cells. Understanding how RNA and other biologically active molecules are transferred and become functional inside target cells is essential for explaining their effects on cellular behaviour.
The diversity of extracellular vesicles also makes this field particularly complex. Vesicles originating from different cell types may contain distinct molecular components and perform different biological functions. Even vesicles released by the same cell type may differ depending on the physiological conditions of the originating cells.
Consequently, studying the mechanisms that control exosome production, cargo selection, release, targeting and delivery will be essential for understanding their contribution to cellular communication and immune regulation.
Exosomes as Potential Biomarkers in Disease
One of the most promising applications of exosomes concerns their potential use as disease biomarkers. Because these vesicles originate from specific cells and tissues, their surface proteins and internal molecular components may provide information about their cellular source.
Certain surface markers can be associated with particular cell types, including prostate cells, melanocytes, cardiomyocytes and syncytiotrophoblasts. Identifying these markers may help researchers determine the origin of extracellular vesicles present in biological fluids.
This characteristic is particularly interesting in cancer research. Malignant cells can release larger quantities of exosomes than healthy cells, meaning that tumour-derived vesicles may be detected in blood and other biological fluids. Their molecular composition may also reflect characteristics of the tumour from which they originate.
Tumour-derived exosomes are not simply passive products of cancer cells. They can actively participate in disease development and influence the surrounding environment. For example, exosomes released by melanoma cells may contribute to preparing lymph nodes for the establishment of metastases. This suggests that extracellular vesicles can participate in the processes that facilitate the spread of cancer.
In addition, the RNA content of exosomes may change during tumour development. These molecular changes could provide useful information about the biological characteristics of malignant cells and potentially serve as indicators of disease progression.
In the future, exosome-based biomarkers could offer more specific and sensitive diagnostic possibilities. For example, they may eventually provide alternatives or complementary information to existing markers such as prostate-specific antigen (PSA) in prostate cancer diagnosis.
Research has also investigated the potential diagnostic value of exosomes in prostate cancer, ovarian cancer, melanoma and brain tumours. Beyond oncology, their molecular composition may provide information relevant to autoimmune and inflammatory diseases, as well as certain cardiovascular conditions.
Another possible application involves the detection of oncogenic mutations through the analysis of genetic material carried by extracellular vesicles. Such approaches could potentially support the identification of molecular abnormalities associated with cancer.
Furthermore, monitoring changes in exosomal content during treatment may help evaluate therapeutic responses. By analysing the molecular composition of vesicles over time, researchers may obtain information about changes occurring within diseased tissues.
Therapeutic Applications of Exosomes
In addition to their potential diagnostic applications, exosomes are being investigated as possible tools for the development of new therapeutic strategies. Their natural capacity to transport biological molecules between cells makes them interesting candidates for delivering therapeutic substances to specific tissues.
Some approaches are based on methods originally developed for loading liposomes with biological compounds. These techniques may be adapted to introduce selected molecules into extracellular vesicles. Research has also explored the use of vesicles in therapeutic approaches for conditions such as familial amyloidosis.
The treatment of paroxysmal nocturnal haemoglobinuria (PNH) provides another example discussed in the context of therapeutic strategies involving biological targeting and the regulation of disease-related mechanisms.
One particularly interesting possibility is to load exosomes with different types of therapeutic molecules. These may include signalling molecules, specific DNA sequences or inhibitory RNA molecules capable of reducing the expression of selected genes.
Such strategies could potentially be used to modify gene activity in hereditary diseases or tumours. By delivering molecules that influence gene expression, exosomes may provide a way to alter the biological behaviour of recipient cells.
Their surface proteins are also important in this context. Specific molecules located on the exosomal membrane may help direct vesicles towards cells or organs expressing the corresponding receptors. This targeting capacity could contribute to improving the delivery of therapeutic substances to diseased tissues.
Because exosomes can enter recipient cells and transfer functional RNA or DNA, they may be capable of modifying the activity of target cells. This property is particularly relevant to the development of gene therapy approaches.
Research groups at Harvard Medical School and Umeå University have demonstrated the possibility of introducing adenovirus-based gene therapy vectors into exosomes. This approach was considered promising because it could potentially improve the precision of therapeutic targeting, facilitate intracellular delivery and reduce unwanted side effects.
Another therapeutic strategy involves targeting the production of exosomes by malignant cells. Since tumour-derived vesicles can contribute to the establishment of favourable conditions for cancer development and metastasis, reducing or counteracting their production could potentially interrupt some of the mechanisms through which cancer cells protect themselves and influence surrounding tissues.
A Rapidly Developing Field of Research
The scientific interest in exosomes has increased considerably over the years. A search of the PubMed database using the terms “exosomes” and “function” identified more than 1,200 publications at the time described in the original article.
The number of publications illustrates the rapid development of this research field. In 2011, approximately 202 publications were identified, compared with 65 in 2006 and only 12 in 2001. This increase reflects the growing recognition of extracellular vesicles as important components of cellular communication and potential tools for medical applications.
Despite this progress, knowledge about exosomes remains incomplete. Many questions concerning their formation, molecular composition, biological functions and therapeutic potential still require investigation. Nevertheless, researchers increasingly recognise their importance in physiological and pathological processes.
The growing interest in this field has also encouraged international scientific collaboration and the development of specialised research organisations. In the cardiovascular research field, a review article was expected in early 2014 to address the emerging knowledge about extracellular vesicles and their functions.
The International Society for Extracellular Vesicles (ISEV) was also established to support research, scientific exchange and collaboration in this rapidly expanding area. The society publishes the Journal of Extracellular Vesicles, providing a specialised platform for communicating research findings.
Overall, extracellular vesicles, and particularly exosomes, represent an important area of investigation in modern cell biology and medical research. Their ability to transport proteins, RNA and other biological molecules between cells gives them a central role in intercellular communication. Their involvement in immune regulation, cancer development, cardiovascular processes and other biological mechanisms also highlights their potential clinical importance.
Further research into their molecular composition, biological functions and mechanisms of action may contribute to the development of new diagnostic tools and therapeutic strategies. Although many challenges remain, the study of exosomes offers promising possibilities for improving the understanding, detection and treatment of various diseases.
