The discoveries of exosomes and tunneling nanotubes that allow intercellular transport of proteins, RNA and DNA among neurons and glia change how we think about neuronal communication.
For many decades we have held the view that much of the inter-cellular activity that goes on in the brain is about electrical signals and their transmission across chemical synapses. Yet the communication and sharing among neurons (and glia) is turning out to be far more vibrant than we had once suspected.
Exosomes: vesicles transporting cargo between cells
In 1983 two separate labs published papers within a week of each other showing the presence of vesicles in reticulocytes (red blood cells) that were ~30-100 nm in diameter carrying a receptor called transferrin that was released into the extracellular space [1, 2]. These vesicles were called exosomes and were the first evidence that cells of any type could shuttle stuff other than neurotransmitters out of the cell. The initial thinking was that this was a way for the cell to jettison proteins it didn’t want. Yet remarkably it was later found that these cargo carrying vesicles were not just dumping stuff in the extracellular space but actually using these exosomes to transport cargo to specific cells [3]. Initially it was thought that exosomes only carried proteins but later it was discovered that they could carry all sorts of other stuff like growth factors and metabolites. And then it got even better. In 2007 it was found that they were transporting mRNA between cells [4] and in 2013, even double stranded DNA [5]. As we know now, cells can pass along all kinds of stuff to each other through exosomes.
Tunneling Nanotubes (TNTs) for inter-cellular transfer of organelles
In 2004 came another discovery. Using PC12 cells (a neuroblastoma cell line from rat adrenal medulla) a German group observed thin membrane encased connections between cells that were just a few 100 nm in diameter (about a tenth of the size of an axon) through which they observed the transport of vesicles moving along at a rate of about 20-30 nm/s [6]. They called them tunneling nanotubes or TNTs. These narrow passageways could extend long distances joining cells to one another and were soon found between all kinds of other cells including between neurons and glia. Sometimes they could last for periods as short as 15 mins and other times up to several hours [7].
Image of Tunneling Nanotube between PC12 cells (from [6]
Implications of Intercellular Cargo
The implications of this intercellular cargo are immense: If neurons and glia can share proteins and genetic material, they can influence one another in more fundamental ways than is possible through signaling with neurotransmitters. Rather than simply setting in motion a response in another cell, they can change the way the other cell behaves and even its capabilities at a more basic level, by changing which proteins are synthesized in another cell for example. The ‘plasticity’ or the physiological changes that can be induced with such sharing are more far reaching – a broader kind of ‘metaplasticity’, going as far as to change the phenotype of the cell. The contents of Oligodendrocyte exosomes, for example, alter gene expression, signal transduction and firing rates in neurons [8-10].
Some Speculation on the Why and How
So why and how would neurons and glia share proteins and RNA? Here are some thoughts (some with evidence and some which are wild speculation, which I’m taking liberty with since this is only a blog post). There are three different paradigms of such sharing that I can think of. Let’s call them generosity, influence and trade.
- Generosity: This involves the sharing of material without something in return that serves some kind of altruistic purpose such as in response to a distress signal. For example, after ischemic events when metabolic resources become limited one study showed that exosomes carrying miRNA released from astrocytes facilitate recovery in neurons and improve neuroplasticity [11]. There is also evidence for sharing of immune response proteins between cells and for the sharing of the mRNA for Arc which is an essential protein for long term memory.
- Influence: The flip side of such ‘giving’ is vying for influence that is designed to take over the functioning of the system rather than serving its greater good. Indeed, it has been found that viruses and tumor cells make use exosomes and TNTs to proliferate their genes. Also prions. HIV for example, can induce the formation of TNTs and viruses have been shown to proliferate through cells through exosomes. RNA carrying cancer genes have also been found in exosomes.
- Trade: This would involve some kind of quid pro quo and while there is no evidence as yet of such an exchange it doesn’t mean it’s not there. For instance, the acceptance of an exosome’s cargo might be contingent on sending back something. If neurons, or all cells in general, had some kind of currency it might be some kind of common metabolic molecule that could be used to ‘pay’ for certain protein or RNA content. Further, given that oligodendrocytes have been found to transfer exosomes in response to neurotransmitter signaling, one could even imagine that the signaling between neurons might serve in some way to negotiate a trade of mRNA or proteins. Now I’m getting carried away, but it does make you wonder if all the spontaneous activity that has nothing to do with any immediate external stimulus is chatter between neurons about what they want from each other to build or make something.
Exosomes and TNTs in Humans
Drug targets that affect the formation of exosomes and TNTs are now being trialed for a host of disorders from cancer to Alzheimer’s. Sadly, however, it is exceptionally difficult, if not impossible, to study these phenomena directly in humans unless perhaps you are using biopsied tissue samples. This makes the success of trials more of a challenge. However, beyond drug targets, what is interesting is that these exosomes can cross the blood brain barrier sending materials made in the brain to other parts of the body. Looking in the CSF for what these are might prove to be tremendously valuable for diagnostics and for understanding the mind-body connection. These phenomena may also contribute to the genetic mosaic of our brains and the diversity among us.
References
[1] C. Harding, J. Heuser, and P. Stahl, “Receptor-mediated endocytosis of transferrin and recycling of the transferrin receptor in rat reticulocytes,” J Cell Biol, vol. 97, no. 2, pp. 329-39, Aug 1983.
[2] B. T. Pan and R. M. Johnstone, “Fate of the transferrin receptor during maturation of sheep reticulocytes in vitro: selective externalization of the receptor,” Cell, vol. 33, no. 3, pp. 967-78, Jul 1983.
[3] M. Basso and V. Bonetto, “Extracellular Vesicles and a Novel Form of Communication in the Brain,” Front Neurosci, vol. 10, p. 127, 2016.
[4] H. Valadi, K. Ekstrom, A. Bossios, M. Sjostrand, J. J. Lee, and J. O. Lotvall, “Exosome-mediated transfer of mRNAs and microRNAs is a novel mechanism of genetic exchange between cells,” Nat Cell Biol, vol. 9, no. 6, pp. 654-9, Jun 2007.
[5] R. Kalluri and V. S. LeBleu, “Discovery of Double-Stranded Genomic DNA in Circulating Exosomes,” Cold Spring Harb Symp Quant Biol, vol. 81, pp. 275-280, 2016.
[6] A. Rustom, R. Saffrich, I. Markovic, P. Walther, and H. H. Gerdes, “Nanotubular highways for intercellular organelle transport,” Science, vol. 303, no. 5660, pp. 1007-10, Feb 13 2004.
[7] X. Wang, N. V. Bukoreshtliev, and H. H. Gerdes, “Developing neurons form transient nanotubes facilitating electrical coupling and calcium signaling with distant astrocytes,” PLoS One, vol. 7, no. 10, p. e47429, 2012.
[8] D. Frohlich et al., “Multifaceted effects of oligodendroglial exosomes on neurons: impact on neuronal firing rate, signal transduction and gene regulation,” Philos Trans R Soc Lond B Biol Sci, vol. 369, no. 1652, Sep 26 2014.
[9] C. Fruhbeis et al., “Neurotransmitter-triggered transfer of exosomes mediates oligodendrocyte-neuron communication,” PLoS Biol, vol. 11, no. 7, p. e1001604, Jul 2013.
[10] C. Fruhbeis, D. Frohlich, W. P. Kuo, and E. M. Kramer-Albers, “Extracellular vesicles as mediators of neuron-glia communication,” Front Cell Neurosci, vol. 7, p. 182, Oct 30 2013.
[11] H. Xin et al., “MiR-133b promotes neural plasticity and functional recovery after treatment of stroke with multipotent mesenchymal stromal cells in rats via transfer of exosome-enriched extracellular particles,” Stem Cells, vol. 31, no. 12, pp. 2737-46, Dec 2013.
