Role of Exosomes in Intercellular Signaling, Maintenance of Normal Physiology, and their Therapeutic Potentials

Cheng-Chang Tsai1, Chen-Han Yang1 and Nan-Shan Chang1-3* 1Institute of Molecular Medicine, National Cheng Kung University Medical College, Tainan, Taiwan 2Department of Neurochemistry, New York State Institute for Basic Research in Developmental Disabilities, New York, NY, USA 3Department of Neuroscience and Physiology, State University of New York Upstate Medical University, Syracuse, NY, USA *Corresponding author: Nan-Shan Chang, Institute of Molecular Medicine, National Cheng Kung University Medical College, Tainan, 70101, Taiwan, Tel: 06-2353535; E-mail: changns@mail.ncku.edu.tw


Commentary
Exosomes are mainly derived from intracellular multivesicular bodies and can be secreted to the extracellular spaces, which has been known for the past two decades [1][2][3][4]. Exosomes have been implicated in intercellular communications, maintaining normal cellular physiology and, importantly, potentials roles in clinical therapeutics. Many studies have documented the compositions of exosomes. Namely, exosomes are rich in specific sets of proteins, DNAs, RNAs, and lipids [5]. Exosomes are vesicles of 40-100 nm in diameter. They are normally released from healthy cells, and are present in diverse body fluids including semen, blood, urine, cerebrospinal fluid, and etc. After binding to specific recipient cells, exosomes may dock on membrane, directly fuse with plasma membrane, and are then endocytosed [1][2][3][4].
Unlike the spherical exosomes, live migrating cells may discard a portion of membrane/cytoskeleton materials. These materials are amphorous and adherent, and can be greater than 10 µm in one of the longest diameters. Yet, they are not the ghosts of dead cells. Despite their large sizes, visiting neighbor cells can engulf them completely. Later on, these fed cells secrete or dump another big chuck of amphorous membrane/cytoskeleton materials, and again other migrating cells phagocytose these adherent materials. This type of cell behavior can readily be observed by time-lapse microscopy. Nonetheless, the molecular nature of this type of exosomes is largely unknown and remains to be characterized.
Cancer cells also embark on intercellular communications, in part, via exosomes to enhance their invasiveness. A recent elegant study showed that fibroblast-secreted exosomes promote breast cancer cell protrusive activity and motility via WNT-Planar Cell Polarity (PCP) signaling [6]. The WNT-PCP pathway controls the distribution of cells within a plane. Many proteins are involved in the signaling, including WNTs Frizzled (FZD) receptors, Van-Gogh-like (Vangl), Dishevelled (Dvl), Smurf, and Prickle-like proteins (PKs), which are needed for cancer metastasis. By mass analysis, CD81 was found to be a key component in the exosomes to direct the breast cancer cell invasiveness. The study appears to provide a link from fibroblastderived exosomes to mobilize autocrine WNT-PCP signaling to drive the invasive behavior of breast MDA-MB-231 cancer cells. That is, breast MDA-MB-231cancer cells endocytose CD81-positive exosomes, and then secrete new exosomes possessing Wnt11 and CD81 markers. Once uptake of these Wnt11-and CD81-expressing exosomes by other breast cancer cells, these cells become activated or polarized due to the autocrine WNT-PCP signaling to exhibit their protrusions, mobility and metastasis. Whether these cells migrate collectively or individually in a directional or non-directional manner is unknown. These experiments were carried out directly using purified exosomes from conditional culture supernatants to feed breast MDA-MB-231 cancer cells and to show the effect in vitro. Indeed, the CD81/Wnt11exosomes are like floating rafts, and their guidance for MDA-MB-231 cells to move along is not directional. Unfortunately, there was no study using time-lapse microscopy to show one cell secretes exosomes and these exosomes are endocytosed by another cell in real time. MDA-MB-231 cells, which were used in the experiments, are very keen to their surrounding environment. They can migrate to a specific, friendly spot in a specific organ(s), where they dock and build a new niche or, they can migrate in a retrograde manner to keep away from an unfriendly or a dangerous environment, as shown in time-lapse microscopy.
The beauty of this study is that CD81/Wnt11-exosomes can be utilized as vehicles to carry therapeutic drugs for targeting cancer ( Figure 1). Conceivably, a specific drug is designed to bindCD81/ Wnt11-exosomes in vivo. Once the drug-tagged CD81/Wnt11exosomes bind cancer cells, the drug will block stromal cells to release new exosomes, and meanwhile kills cancer cells. Without the signal released from stromal cell, the tumor cell's migration will decrease. Exsosomes are also known to play a role in the communications between neurons and glials in vivo [7]. Another recent elegant study reported that myelinating oligodendrocytes secrete exosomes to support the normal physiological functions of neurons [8]. The authors demonstrated that glutamate derived from neurons acts as a neurotransmitter to induce oligodendrocytes to release exosomes ( Figure 2). Experiments were designed to show the involvement of activation of ionotropic glutamate receptors in oligodendrocytes to cause horizontal transfer of exosomes from the oligodendrocytes to neurons. Supporting evidence revealed that glutamate secreted by neurons drives the influx of extracellular Ca 2+ to oligodendrocytes via NMDA and AMPA receptors. These cells then secrete exosomes, which are endocytosed by neurons. The exosomes appear to be intended for sustaining the normal neuronal metabolism even under stress conditions (oxidative stress and starvation) [8]. It would be of interest to see whether oligodendrocyte-derived exosomes express CD81/Wnt11. Nonetheless, formation of exosomes in oligodendrocytes does not necessarily require neuronal signals. While the study was elegantly designed, what remains unclear is how exosomes are delivered to neurons. Experiments were designed to isolate exosomes from the condition media of cultured oligodendrocytes, and the isolated exosomes were added to the neuron culture. Whether exosomes travel through the myelin sheath of oligodendrocytes and are then endocytosed by neurons in vivo is not clear. It is reasonable to postulate that neuron picks up exosomes via its soma or myelineated axon.
In summary, many salient features of exosomes have been reported, especially in intercellular signaling and communications and functional supports among cells [1][2][3][4][5]. However, the contents of exosomes are highly complicated. The key components that support signaling and normal cell functions can be subjected to debate and intensive investigations. Nonetheless, once properly designed, exosomes are nice vehicles for targeted drug delivery in vivo.