The extracellular matrix is an organized network of fibres that acts as a support structure to cells and directly influences their behaviour. The interaction between a cell and the extracellular matrix is crucial to tissue-specific cell behaviour. Indeed, scientists have manipulated this cell-matrix relationship by using biomaterials to create environments that can facilitate cell growth, to create tissue for research or transplantation purposes.
The use of human stem cells presents exciting opportunities for tissue engineering. Pluripotent stem cells enable the growth of multiple different cell types, which contain the same genetic code as the person from whom the cells were originally extracted. Stem cells can be directed to grow into a certain cell type by selecting an appropriate extracellular environment – through specialized materials – to provide physical cues that direct growth. Hydrogels are commonly used due to their biocompatibility and tuneable physical properties.
Researchers from the University of Akron have studied the effect of different extracellular environments on promoting differentiation of neural stem cells into neurons (Biomed. Mater. 13 024102). They performed the investigation by using hydrogels with different physical properties, i.e. stiffness and cell binding domains.
The researchers found that neuronal differentiation was promoted when cells were grown on soft surfaces, with a stiffness of 0.1–0.8 kPa being favourable over 4.2–7.9 kPa. In addition, they observed an increased expression of a key neuron protein marker (β-III tubulin), as well as significantly increased neurite growth, in the softer hydrogels (0.1 and 0.8 kPa). The stiffer hydrogels, on the other hand, promoted glial cell differentiation instead of neuronal.
On top of investigating the effect of stiffness on neural stem cell differentiation, the researchers also studied the impact of physical binding of cells to their surrounding environment on differentiation. They established that the amount of binding sites available to cells is important – with differing levels affecting neural differentiation – but not as important as the physical stiffness of the extracellular environment.
To investigate binding, the researchers introduced a specific peptide sequence, Arg-Gly-Asp (RGD), to the synthetic hydrogel used in this study. RGD is a peptide sequence present in multiple biopolymers of the extracellular matrix – fibronectin and collagen, for example. It enables cell binding through multiple integrin receptors at the surface of cells.
This study echoes seminal papers exhibiting that a physical stiffness of below 1 kPa is favourable for neuronal differentiation from stem cells, while stiffness above1 kPa promotes glial cell formation. It builds on previous work by establishing that a small difference in stiffness can switch the lineage of neural stem cells from neuronal to glial, with previous investigations having only tested 1 and 10 kPa hydrogels for neuronal and glial growth.
This research presents a general guideline for differentiating neural stem cells to neuronal cells, which will aid engineering of brain tissue in future. This information could be manipulated to create multicellular brain constructs from patient stem cells, potentially creating translational and stratified models for medical research in the lab. The authors suggest that an interesting progression from their study would be to investigate the cell-to-cell communication and interactions within the neural stem cell niche, as opposed to the cell-to-extracellular interactions exhibited here.
