Northwestern University scientists have developed a powerful analytical method to direct stem cell differentiation. Out of millions of possibilities, they rapidly identified the chemical and physical structures that can cue stem cells to become osteocytes, cells found in mature bone.
Scientists can use the method, called nanocombinatorics, to build enormous libraries of physical structures varying in size from a few nanometers to many micrometers for addressing problems within and outside biology.
This method could be used to assess which chemical and physical structures—including size, shape and composition—work best for a desired process or function.
Nanocombinatorics holds promise for screening catalysts for energy conversion, understanding properties conferred by nanostructures, identifying active molecules for drug discovery or even optimizing materials for tissue regeneration, among other applications, the developers say.
The new analytical method utilizes a technique invented at Northwestern called polymer pen lithography, where basically a rubber stamp having as many as 11 million sharp pyramids is mounted on a transparent glass backing and precisely controlled by an atomic force microscope to generate desired patterns on a surface. Each pyramid—a polymeric pen—is coated with molecules for a particular purpose.
In this work, the researchers used molecules that bind proteins found in the natural cell environment, such as fibronectin, which could then be attached onto a substrate in various patterns. (Fibronectin is a protein that mediates cell adhesion.) The team rapidly prepared millions of textured features over a large area, which they call a library. The library consisted of approximately 10,000 fibronectin patterns having as many as 25 million features ranging in size from a couple hundred nanometers to several micrometers.
To make these surfaces, they intentionally tilt the stamp and its array of pens as the stamp is brought down onto the substrate, each pen delivering a spot of molecules that could then bind fibronectin. The tilt results in different amounts of pressure on the polymeric pens, which dictates the feature size of each spot. Because the pressure varies across a broad range, so does the feature size.
The researchers then introduced mesenchymal stem cells, or MSCs, to the library of millions of fibronectin features.
The researchers found areas with stem cell differentiation and areas with none. Nanoscale features, particularly protein spots that were 300 nanometers in diameter, were more likely to lead to bone-like cells than larger micron-scale features.
The researchers next built a library made up of only 300-nanometer dots and introduced stem cells. Almost all of the cells became bone-like.
This stem cell differentiation was accomplished without the use of additional chemical cues (beyond the proteins in the patterns). The transition from stem cell to osteocyte was dictated solely by the physical cues of the patterned structures. And the researchers demonstrated better control over stem cell differentiation than chemical reagent methods currently used.
The research was published in the Proceedings of the National Academy of Sciences.
