Cytocentric Visionaries: Donald Phinney, Chair Department of Molecular Therapeutics, The Scripps Research Institute
Part One: Mesenchymal Stem Cells, as Healers, Aren’t So Selfless
Chair of the Molecular Therapeutics Department at Scripps, Dr. Phinney has almost 20 years of experience in the adult stem cell field and has been an editor of prominent stem cell journals. Here, Alicia Henn talks with Dr. Phinney about his latest publication in Nature Communications, ”Mesenchymal stem cells use extracellular vesicles to outsource mitophagy and shuttle microRNAs”[Phinney et al., 2015]. This conversation on understanding how MSC stem cells affect macrophage biology was edited for length.
Phinney and his co-authors reported that in response to oxidative stress, MSCs export damaged mitochondria to nearby macrophages, which recycle them for their own bioenergetics. The MSCs also export exosomes containing miRNAs that reduce the inflammatory activities of macrophage through TLR-7 signaling pathways. This benefits the nearby MSCs, and is also a mechanism by which MSCs may have their clinical effects on surrounding tissues.
Thank you for talking with me today. I have to say that your recent paper in Nature Communicationschanged the way I think about cellular communication. In addition to cells being able to send proteins back and forth, cells can send out whole organelles that modify other cells. That’s mind-blowing.
DP: Thank you, but others have described RNA and mitochondrial transfer before us. What has intrigued me for years is the idea that MSC are therapeutic through an altruistic mechanism. There are thousands of papers that MSC can cure disease. My interest was always, why would an MSC do that? Cells are programmed to survive.
What really got me excited about that work is that we are starting to understand the physiological mechanism by which MSC might modulate other immune cells to promote their own survival. That makes the most sense to me from a basic biology point of view. MSCs may be immunomodulatory and anti-inflammatory and pro-angiogenic, but they do all these things to promote their own survival under oxidative stress.
Your data beautifully showed that MSCs, in response to stress export partially depolarized mitochondria and exosomes, to the benefit of nearby macrophage.
DP: Yeah that’s a cool aspect of that study. We postulated that when the partially depolarized mitochondria transfer from MSCs to macrophages, the macrophages are able to reutilize some of those mitochondria for their own benefit. So it is mutually altruistic.
The key is that the exosomes prevent the macrophage from being activated. Activation would be detrimental to the MSC. We over-expressed one of the microRNAs from the exosomes and showed that it suppressed macrophage activation in response to silica. So there’s evidence that those exosomes are potently affecting macrophage biology.
The interesting thing is that we purified exosomes from five different human donors and analyzed their microRNA content and it was remarkably homogeneous across exosomes from different donors. That’s really intriguing.
It says that these exosomes are homogenous across different MSCs. Their content is designed or evolved for a particular purpose. And maybe a major purpose is to modulate innate immune cell function. This gets back to providing a rationale as to why MSCs are so immunomodulatory.
Technically, what was the toughest part of carrying out these experiments?
DP: The hardest part was the microscopy. Luckily, University of Pittsburgh has a top-notch microscopy core. The time-lapse photography was really critical in terms of trying to understand vesicle transport and uptake and cell-cell interactions. The biochemistry was also somewhat difficult. Producing large numbers of MSC to collect exosomes was another obstacle. It is not an easy thing to actually purify exosomes, if you do it right. There are a lot of microvesicles of different sizes so getting high-quality exosome preps was pretty difficult.
In Part Two, we continue our discussion with Dr. Phinney about MSCs vs. Exosomes and Therapeutics.
Phinney, D. G., et al. (2015), Mesenchymal stem cells use extracellular vesicles to outsource mitophagy and shuttle microRNAs, Nat Commun, 6, 8472.
About the Author
Alicia D Henn, PhD, MBA
Alicia Henn has been the Chief Scientific Officer of BioSpherix, Ltd for two years. Previously, she was a researcher at the Center for Biodefense Immune Modeling in Rochester, NY. Alicia obtained her PhD in molecular pharmacology and cancer therapeutics from Roswell Park Cancer Institute in Buffalo, NY and her MBA from the Simon School at University of Rochester in Rochester, NY.