UCSD scientists devise tiny sensors that detect forces at cellular level

A big focus of stem cell research is trying to figure how to make a stem cell specialize, or differentiate, into a desired cell type like muscle, liver or bone. When we write about these efforts in the Stem Cellar, it’s usually in terms of researchers identifying proteins that bind to a stem cell’s surface and trigger changes in gene activity inside the cell that ultimately leads to a specific cell fate.

But, that’s not the only game in town. As incredible as it sounds, affecting a cell’s shape through mechanical forces also plays a profound role in gene activity and determining a cell’s fate. In one study, mesenchymal stem cells would specialize into fat cells or bone-forming cells depending on how much the MSCs were stretched out on a petri dish.

An artist’s illustration of nano optical fibers detecting the minuscule forces produced by swimming bacteria. Credit: Rhett S. Miller/UC Regents

Since we’re talking about individual cells, the strength of these mechanical forces is tiny, making measurements nearly impossible. But now, a research team at UC San Diego has engineered a device 100 times thinner than a human hair that can detect these miniscule forces. The study, funded in part by CIRM, was reported yesterday in Nature Photonics.

The device is made of a very thin optical fiber that’s coated with a resin which contains gold particles. The fiber is placed directly into the liquid that cells are grown in and then hit with a beam of light. The light is scattered by the gold particles and measured with a conventional light microscope. Forces and even sound waves caused by cells in the petri dish change the intensity of the light scattering which is detected by the microscope.

Donald Sirbuly,
team lead

In this study, the researchers measured astonishingly small forces (0.0000000000001 pound of force, to be exact!) in a culture of gut bacteria which swim around in the solution with the help of their whip-like flagella. The team also detected the sound of beating heart muscle cells at a level that’s a thousand times below the range of human hearing.

Dr. Donald Sirbuly, the team lead and a professor at UCSD’s Jacobs School of Engineering is excited about the research possibilities with this device:

“This work could open up new doors to track small interactions and changes that couldn’t be tracked before,” he said in a press release.

Bradley Fikes, the biotechnology reporter for the San Diego Union Tribune, reached out to others in the field to get their take on potential applications of this nanofiber device. Dr. John Marohn at Columbia University told Fikes in a news article (subscription is needed to access) that it could help stem cell scientists’ fully understand all of the intricacies of cell fate:

“So one of the cues that cells get, and they listen to these cues to decide how to change how to evolve, are just outside forces. This would give a way to kind of feel the outside forces that the cells feel, in a noninvasive way.”

And Eli Rothenberg at NYU School of Medicine, also not part of the study, summed up the device’s novelty, power and ease of use in an interview with Fikes:

“One of the main challenges in measuring things in biology is forces. We have no idea what’s going on in terms of forces in cells, in term of motion of molecules, the forces they interact with. But these sensors, you can put anywhere. They’re tiny, you can place them on the cells. If a cancer cell’s surface is moving, you can measure the forces…The fabrication of this device is quite straightforward. So, the simplicity of having this device and what you can measure with it, that’s kind of striking.”



Tunable hydrogels guide stem cell differentiation

Differentiating stem cells into mature cells of adult tissue involves many intricate steps to get them to develop into the right cell types. You could compare the process to the careful adjustments you make when tuning a guitar.

In the body, stem cells receive cues from their surrounding environment to mature into specific types of cells. These cues can be biochemical – molecules like lipids, growth factors and metabolites (products of cell metabolism) – or they can be physical – the stiffness of surrounding tissue. But these molecules and structures aren’t always present when scientists attempt to differentiate stem cells outside the body, say in a cell culture dish.

One way researchers are improving the methods for differentiating stem cells outside the body is by using biomaterials such as hydrogels that mimic properties of the structures and molecules found naturally in various stem cell niches that aid in their maturation to adult cell types.

A CIRM-funded study published last week in the journal Chem, has developed “tunable hydrogels” that direct stem cells to differentiate into brain, cartilage and bone cells based on adjustments to the hydrogel’s stiffness and metabolite profile. The work was a collaboration between scientists in New York and in Scotland and one of the co-authors, Bruno Péault, was a CIRM-funded scientist in California during the time of the study.

Hydrogels with different stiffness' direct stem cells to differentiate into different types of tissue. (Chem)

Hydrogels with different stiffness’ direct stem cells to differentiate into different types of tissue. (Chem)

Tuning gels to differentiate stem cells

The scientists started with hydrogels composed of nanofibers that varied in stiffness and observed that perivascular stem cells (from the connective tissue surrounding blood vessels) grown in more flexible gels turned into brain cells and those that were grown in stiffer gels turned into bone cells. The stiffness of these different hydrogels was comparable to that of actual brain and bone tissue, which indicated that stiffness is important for stem cell fate.

But stiffness alone isn’t responsible for directing stem cells into different cell fates – biochemical metabolites are also key to this process. The authors also analyzed the metabolite profiles of the different hydrogels to determine which metabolites were important for stem cell differentiation. They tested different concentrations of over 600 metabolites in the hydrogels during stem cell differentiation and found that certain lipids like lysophosphatidic acid and cholesterol sulfate were essential for differentiation into cartilage and bone tissue respectively. When these specific lipids were added to regular stem cell cultures (without hydrogels), the stem cells differentiated towards cartilage and bone cells. Thus they concluded that both the metabolite profile and the stiffness of hydrogels are important for directing stem cell differentiation.

Interestingly, the authors also showed how metabolites like cholesterol sulfate could influence and activate transcription factors – proteins that also direct stem cell differentiation – which controlled the activation of bone-related genes. This finding suggests a relationship between metabolite expression and transcription factor activity and offers a simpler way to activate transcription factors important for stem cell fate.

Big picture of tunable hydrogels

Looking at the big picture, this study offers a useful strategy to identify molecules that promote formation of specific tissue types from stem cells. These molecules could be potential drug candidates that could aid in regenerating bone and cartilage tissue for patients with osteoporosis or osteoarthritis.

Co-senior author on the study and professor at the University of Glasgow, Matthew Dalby, who was interviewed by Science Magazine elaborated on the importance of their study:

Matthew Dalby

Matthew Dalby

“Our ambition is to simplify drug discovery — by using the cell’s own metabolites as drug candidates. For example, cholesterol sulfate, which our rigid gel revealed as critical to bone cell differentiation, could be a safer solution (e.g., minimal off-target effects) for treating osteoporosis, spinal fusion, and other bone-related conditions. Presently, growth factors are used, but these can lead to unwanted collateral damage, and government agencies in the UK and US have published warnings against their use.”

Rein Ulijn, co-senior author with Dalby and professor at the City University of New York and University of Strathclyde, concluded by emphasizing how the metabolites they identified could be potential drug candidates and would pass regulatory approval if shown to be safe and effective:

Rein Ulijn

Rein Ulijn

“That you can use simple metabolites like cholesterol sulfate, which is readily available, to induce differentiation is in my view very powerful if you think about this as a potential drug candidate. These metabolites are inherently biocompatible, so the hurdles to approval are going to be much lower compared to those associated with completely new chemical entities.”

In the future, both teams plan to further “tune” their hydrogels to mimic more complex tissue environments that incorporate additional properties besides stiffness in hopes of creating more relevant 3D micro-environments to model the stem cell niche.