The “Simple” Capillary Finds Its Niche

Today we feature guest writer, Kim Smuga-Otto, stem cell biologist and assistant researcher in the Regenerative Biology Laboratory at the Morgridge Institute for Research at the Wisconsin Institutes for Discovery.

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When I was a child, I was taught that arteries were red, veins were blue, and in between them spread a net of tiny tubes called capillaries that, the text assured me, managed to reach all the cells in my body. The capillaries started off red and went to blue as they exchanged oxygen and nutrients for carbon dioxide and waste. The Wow factor—that the vessels were so small that cells, something so tiny you need a microscope to see, had to squeeze through one at a time—made an impression on my developing geek brain. But once you get past that, it’s mostly just plumbing. So as I expanded my knowledge of biological, the circulatory system remained a comfortably simple diagram of red, blue and tiny tubes.

Turns out, there’s more to their story.

According to Dr. Shahin Rafii, whose presentation at the Wisconsin Stem Cell Symposium I attended on April 10, 2013, capillaries and their endothelial cells play a critical role inducing stem and progenitor cells to proliferate and replace damaged tissue. He referred to them as a possible “magic bullet”, whose transplantation could promote organ regeneration.

Dr. Rafii’s talk was filled with these kinds of analogies—endothelial cells had “zip codes” and could “tweet” to the rest of the body—and ambitious pronouncements. It made for a riveting and exciting talk, far more engaging than your average weekly lab meeting with Excel bar graphs representing gene expression. I especially enjoyed Dr. Rafii’s diatribe against the evils of serum in cell culture media.* As my first try at conference blogging, it was clear which presentation I wanted to write about.

But the speaker’s style aside, the content of his talk felt solid. And as I went through his publications, I found a strong case for the critical role endothelial cells play in our biology.

How can something as simple as a tube of cells, a single layer thick, be critical to organ regeneration? The key, points out Dr. Rafii, lies in the cells’ diversity and in their close proximity to the stem and progenitor cell niches in the body. It turns out that the endothelial cells of your liver are different from the ones in your lungs and different still from the ones that permeate your bones. They may present the same receptors for activation, but the resulting release of growth factors (Dr. Rafii refers to these as angiocrine factors) are unique to the particular niche the endothelial cells are interacting with.

And now, for the non developmental biologists, a bit of background. Niches are organized clusters of cells where the magic of regeneration begins. As an embryonic stem cell biologist, my default mindset is that my cell culture is either dividing (good) or differentiating (usually bad). But in real life, inside me, a progenitor cell divides asymmetrically, giving rise to a copy of itself and a cell that’s already on the pathway to differentiating. It would seem to be an inefficient process, but the tradeoff is a highly regulated system. Unregulated, ever dividing cells have a name—cancer.

Dr. Rafii’s research points to endothelial cells and their angiocrine factors being the activators of these niches. His supporting experiments are fairly straightforward. Damage the target organ so that it must be regenerated. Disable the associated endothelial cells, usually by eliminating their primary receptor, and confirm that regeneration does not take place. Finally, restore functioning endothelial cells to the body and observe organ regeneration.

Of course, the devil is in the details.

Most critical is knowing you have the right cells. In his 2009 paper published in the journal Cell Stem Cell, Dr. Rafii’s group detailed their approach to staining cells of the bone marrow to identify a subclass of endothelial cells by their surface receptors, a receptor combination that was different than other blood vessel cells or other progenitor cells could conceivably play the critical role in inducing regeneration. They also used mice with green fluorescent reporter genes driven by the same promoter of the primary endothelial cell activation receptor. So now when their endothelial cells were activated, and producing the receptor, the cells would light up green.

The final tool in Dr. Rafii’s arsenal was a mouse line where the critical receptor could be eliminated from its genome. This inducible system means that the mouse’s endothelial cells were fully functional during development, critical to a live birth, but could be deleted from the DNA by the introduction of an otherwise harmless drug. Dr. Rafii showed that under normal conditions ( i.e., full and functioning organs), this “knocked out” receptor goes unnoticed.

Armed with these tools, they gave mice a sublethal dose of irradiation, ridding them of blood cells. The radiation was not enough to kill the blood stem cells that reside in the bone marrow and under normal circumstances where the mice were protected from infection, the blood cells would be repopulated and the mice would recover. However when the endothelial cells were disabled, either by blocking their receptors with antibodies or using the inducible knockout mice, blood cells were not regenerated. Transplanting bone marrow endothelial cells restored blood cell regeneration, but it couldn’t be just any endothelial cells. Liver endothelial cells had no effect.

This basic approach was repeated in liver and lungs in 2011 and 2012 respectively. A mouse can lose up to 70% of its liver, and, provided the remaining tissue is healthy, the organ can recover. And, while a lung won’t regenerate, removing the left lung of a mouse will result in increased growth of the right lung to compensate for the lost lobes. In both cases, regeneration and growth were not observed in the inducible knockout mice and transplantation of the specific endothelial cells restored the capabilities.

I’ve only given the basic outline of the experiments. The papers go into far greater detail for the controls and the actual biological molecules being affected. I recommend reading the papers in full if you’re interested.

After going through these experiments, Rafii turned to his proposed transplant, a generic endothelial cell type that could be “educated” to a specific type and let loose in the bloodstream to naturally find its proper niche to influence. If that could work, it would be a marvelous discovery. My more pragmatic side is taking a wait and see approach. But regardless, Dr. Rafii’s research has fully dislodged the simple red / blue / tiny tube simplification of capillaries from my head.

* I can’t resist this quote from Rafii when explaining why he doesn’t allow serum in his lab.

“This serum bothers me enough, I have an obsession with serum. Every time as a hematologist, I ask my post doc .. I ask our new dean at Cornell – What is serum? […] Serum is a product of clotted blood. […] And usually you see serum 20 hours after someone is deceased. It doesn’t exist in biology.”

He’s right you know.

References

1. Hooper, A.T. et al. (2009) Engraftment and reconstitution of hematopoesis is dependent on VEGFR2-mediated regeneration of sinusodial endothelial cells. Cell Stem Cell. 4, 263–74.
2. Ding, B-S. et al. (2010) Inductive angiocrine signals initiate and sustain regenerative lung alveolarization. Nature 486, 310–15.
3. Ding, B-S. et al. (2011) Endothelial-derived inductive angiocrine signals initiate and sustain regenerative lung alveolarization. Cell 147, 539–53.