At any given moment in the life of an adult stem cell, it receives various signals that prompt it to make decisions. The cell could divide and make another stem cell; it could differentiate—becoming a specialized tissue cell; or it could die or enter a dormant phase. As we get older, says Benjamin D. Cosgrove, Meinig School of Biomedical Engineering, the balance of these decisions goes askew, leaving us with fewer well-performing stem cells and an inability to repair our tissues.
Cosgrove, who studies muscle stem cells, is working to understand this decision-making process, to model it, and to propose solutions to help our muscles age better and heal faster from injury. His research could also lead to therapies for muscular degenerative diseases.
“We think about all of the molecular networks in old or diseased muscle stem cells and ask how these combinations of signals change the performance of the cells,” Cosgrove says. “Once we understand that, can we intervene with drugs or by slightly changing the environment that supports the cells? Overall, we’re bringing engineering approaches to ask how we can predictably reprogram the stem cells’ fate, such that they act as healthy stem cells.”
Stem Cells: Balancing the Elite, the Adequate, and the Dysfunctional
Muscle stem cells—also known as satellite cells because they appear to orbit the muscle fiber cells—are activated when muscles are damaged, but not all the stem cells pull their weight.
“We’ve found that the stem cells in our muscle tissue are by molecular identity very similar, but if you ask them to do their job, to replenish muscle tissue, their function is quite varied,” Cosgrove explains. “There are elite performing stem cells, okay stem cells, and really dysfunctional cells. We’ve shown that the balance in these populations becomes altered in the aging process.”
As our tissues age, they accumulate a larger population of dysfunctional cells. By improving the performance of these cells or by shifting the balance so that we have more elite performers as we age, researchers may be able to vastly improve muscle regeneration.
First, they need to be able to differentiate the dysfunctional cells from the elite. With collaborators, Cosgrove has identified key molecular markers for the different classes of performers. “When we profile these stem cells, one cell at a time, we find a lot of unexpected variation in what genes they’re expressing, many of which affect critical regulatory pathways controlling cell fate decisions,” Cosgrove says. “And this variation becomes even more pronounced in muscle tissues from elderly mice and humans and muscular dystrophy samples.”
Studying just how these pathways impact cell fate is a challenge, given the rarity of the cells. Cosgrove is tackling this challenge with new materials and technology.
Engineering Better Stem Cell Microenvironments Outside the Body
It’s an idea many researchers are chasing: plucking healthy muscle stem cells from a biopsy of a patient, reprogramming them outside the body, and transplanting the healthy cells back into the patient. The procedure could provide long-term muscle regeneration by increasing the population of elite cells.
One problem is that stem cells are finicky subjects outside the body. “They need a lot of coaxing, a lot of gentle support and just the right stimulation,” Cosgrove says. Researchers in the field have been able to prompt muscle stem cells to proliferate in culture, but the cells lose their identity and no longer know their role when transplanted back into the body.
“There are a number of environmental variables that stem cells normally interact with—the matrix they adhere to, the nutrients and chemicals they need and don’t want too much of, the three-dimensional architecture of their environment, and so on. All of these variables influence every cell in our body, but they are frequently lost as we take cells out and put them in a tissue culture or petri dish,” Cosgrove says. “What we’re finding, along with many others, is that if we don’t give the cells just the right balance of both mechanical and chemical factors that stimulate their proliferation, they specialize and lose their stem-cell identity.”
“We’re bringing engineering approaches to ask how we can predictably reprogram the stem cells’ fate, such that they act as healthy stem cells.”
Cosgrove’s first step towards a solution is to build environments outside the body that more closely resemble the cells’ native environment. “We’ve taken materials engineering approaches to generate just the right substrate: If it’s too stiff, the cells differentiate. If it’s too soft, they won’t divide. If we mimic the mechanical properties of the materials they reside on very carefully, we can tune the rigidity until it is just right.”
Once the substrate is perfect, Cosgrove sandwiches the cells between materials that give them the right three-dimensional architecture and orchestration of molecules on the surfaces—all so the cells feel at home. “It’s a really important challenge because what we’re trying to achieve is a way to take these very rare cells, enhance their potential, and inject them into patients with catastrophic injury or debilitating degenerative diseases,” Cosgrove says. “It’s a long-term cell-engineering challenge but one we’re very optimistic about.”
A Systems Biology Approach to Understanding a Cell’s Fate
Along with the microenvironments, Cosgrove and his lab are working towards a better understanding of the complicated signals that define cell fate decisions—decisions to divide, differentiate, die, or go dormant.
“In the past, there’s been a kind of reductionist approach,” Cosgrove says. “If we knock out a gene or remove a protein, we can reveal how necessary that protein is to some biological function. But we’re finding that therapies relying on this method are becoming less and less successful, because for example in cancer, another mutation arises or resistance develops. Drug development is in some ways stalled by this challenge. What’s becoming more prominent is to think about the biological system intact, as a complicated set of simultaneously moving variables. The solution is to approach the problem as an engineer.”
With tens of thousands of genes, this is no easy task. “So what we do in so-called systems biology is to really think about how to accurately measure the salient molecular variables,” Cosgrove says. “Upon measuring these variables, we build models to understand how the information flow in a cell’s regulatory network combines, splits, and ultimately controls cell decisions.”
For muscle stem cells, Cosgrove has identified about 30 variables that significantly impact the cell’s fate. To make sense of how these variables work together, he relies increasingly on computational algorithms that encode and model. These algorithms take the lab’s measurements of the salient variables and relate them to the ultimate fate and function of the cells.
“These systems approaches are really taking a more comprehensive view to find out the set of molecules we should target with drugs to predictably get the stem cells to do what we want,” Cosgrove says. “We cannot reason our way to a solution, so we need computational models to identify how to push those levers. In doing so, we can generate more specific drug regimes that are less susceptible to adaptation, resistance, or toxicity.”
Cornell’s Systems Biology and Radical Collaboration
In systems biology, taking a more comprehensive view requires more than one expert in the room, Cosgrove says. “We’re bringing engineering solutions to these problems, but we exist at this intersection between different disciplines in ways that blur traditional boundaries,” says Cosgrove.
Cosgrove explains that Cornell is absolutely the best place to do this work. “We have a great set of clinical collaborators at Weill Cornell Medicine. In Ithaca we interact with researchers in Computational Biology, Nutritional Sciences, and the Stem Cell Program in the Vet School. We have an excellent set of biotechnology, animal, and materials facilities,” Cosgrove says. “Of all the places I was looking to start my lab, Cornell had by far the best environment.”
These resources and the diversity of collaborators and students allow for new ways of thinking and the new approaches the field is demanding. “We have to collectively solve these complicated problems that take dedication, a little bit of math, and a lot of collaboration,” Cosgrove says. “We can’t solve this by ourselves, but I think that’s the exciting part—to, with the students and our collaborators, face the complexity and difficulty of regenerative medicine challenges and come up with new solutions, ones that hopefully will have an impact in patients’ lives.”