Well-defined, modifiable hydrogel networks to unravel key parameters that control stem cell fate in the bone marrow.

Two adult stem cell populations reside in the bone marrow: marrow stromal/mesenchymal stem cells (MSCs) and haematopoietic stem cells. The marrow has a unique mechanical environment, which is known to be modulated by physiological activity and disuse, as well as in response to disease and aging. Many cells in the body, including stem cells, are highly mechanoresponsive. Physical characteristic that are known to direct stem cell fate include topography, stiffness and the spacing and clustering of adhesive ligands. For example, culture of MSCs on surfaces of different stiffnesses will direct their fate down neurogenic, myogenic and osteogenic lineages in the absence of soluble factors. However, these phenomena have only been systemically explored in cells cultured on 2D surfaces, which do not replicate the 3D environment of native tissues. There are a number of reports on the bulk mechanical properties of bone marrow that have been determined from measurements of hydrostatic pressure and viscosity. However, very little is known about the local mechanical properties of the marrow at the scale at which a cell mechanically detects the stiffness of its local environment. Moreover, in vitro models that account for these 3D physical properties of the marrow allow researchers to ask fundamental questions on the effect of factors such as stiffness in directing stem cell fate and in the maintenance of haematopoiesis in health and disease.

To address these questions, we will use a combination of fluorescent microscopy and atomic force microscopy (AFM)-based microindentation to characterise the mechanical environment of the stem cell niche in mouse bone marrow. The Gentleman lab has developed a novel PEG-peptide hydrogel, in which systematic modifications of hydrogel chemistry allow for precise control of hydrogel physical properties such as stiffness, adhesive ligand positioning and gel degradability. PEG-peptide hydrogels are also biocompatible, allowing for encapsulation of live cells, making these gels an ideal system to mimic the bone marrow in vitro. Therefore, we will also create PEG-peptide hydrogels with stiffnesses that match those of native bone marrow, encapsulate live cells, and examine how changes in the physical and mechanical properties on the marrow affects stem cell response in native tissue-like niches.

This interdisciplinary project melds expertise in polymer synthesis, mechanobiology, peptide chemistry, stem cell biology and mechanics at KCL, UCL and Imperial to determine the mechanical properties of the bone marrow stem cell niche, and then create an in vitro model based on well-defined, peptide-modified hydrogels with modifiable physical and biological properties that mimics it. With this model, we will ask fundamental questions regarding how stem cells respond to physical properties of their environment. Overall, this project should provide fundamental insights into stem cell mechanobiology and the role of physical properties in the bone marrow stem cell niche in health and disease.