Hydrogel mechanical properties are more important than osteoinductive growth factors for bone formation with MSC spheroids

Abstract

Mesenchymal stromal cells (MSCs) can promote tissue repair in regenerative medicine, and their therapeutic potential is further enhanced via spheroid formation. We demonstrated that intraspheroidal presentation of Bone Morphogenetic Protein-2 (BMP-2) on hydroxyapatite (HA) nanoparticles resulted in more spatially uniform MSC osteodifferentiation, providing a method to internally influence spheroid phenotype. Stress relaxation of hydrogels has emerged as a potent stimulus to enhance monodispersed MSC spreading and osteogenic differentiation, but the effect of hydrogel viscoelasticity on MSC spheroids has not been reported. Herein, we describe a materials-based approach to augment the osteogenic potential of entrapped MSC spheroids by leveraging the mechanical properties of alginate hydrogels. Compared to spheroids entrapped in covalently crosslinked, elastic alginate, calcium deposition of MSC spheroids was consistently increased in ionically crosslinked, viscoelastic alginate. We observed significant increases in calcium deposition by MSC spheroids loaded with BMP-2-HA in viscoelastic gels compared to soluble BMP-2, which was higher than all elastic alginate gels. Upon implantation in critically sized calvarial bone defects, we observed enhanced bone formation in all animals treated with viscoelastic hydrogels. Increases in bone formation were evident in viscoelastic gels, regardless of the mode of presentation of BMP-2 (i.e., soluble delivery or HA nanoparticles). These studies demonstrate that the dynamic mechanical properties of viscoelastic alginate are an effective strategy to enhance the therapeutic potential of MSC spheroids for bone formation and repair.

Human iPSC-Vascular Smooth Muscle Cell Spheroids Demonstrate Size-dependent Alterations in Cellular Viability and Secretory Function

“…To this end, many studies have found that culturing cells in a 3D spheroid environment enhances their therapeutic potential and survival. 3 These 3D spheroids are self-assembled aggregates of cells and more closely resemble the tissue microenvironment. To date, several different stem cell lineages have been formed into spheroids including hiPSCs and mesenchymal stem cells (MSCs) derived from umbilical cord tissue, endothelial cells, hepatocytes, adipose, smooth muscle, bone marrow, and menstrual blood.…”

“…13,16,17 In comparison to the traditional culture technique of cells in a monolayer, cells cultured as spheroids have demonstrated enhanced cell-survival, paracrine factor secretion, and tissue forming potential. 3,4,6,8,9,[18][19][20][21] They also have higher levels of tissue-specific gene expression and increased exosome production. 10,22 Overall, spheroids enhance biological processes relating to immunity, inflammation, angiogenesis, and wound healing.…”

“…2 However, a major obstacle to clinical use of these cells is their reduced therapeutic potential and rapid death following transplantion. [3][4][5][6][7] Other restrictions to direct cell use have been limits in cell expansion, migration from the targeted therapy site, and inability to withstand hostile microenvironments. 8,9 Therefore, there is mounting interest in improving the survival and efficacy of stem cell therapies in vivo.…”

“…7,10 Use of the hiPSC-derived secretome in lieu of cells has allowed researchers to overcome obstacles relating to in-culture cell expansion and post-transplantation cell death. 3,4,6 Additionally, the hiPSC-derived secretome is cell-free and carries less risk of immune reactivity. 11 Previous studies have demonstrated that the hiPSCderived secretome can be used to treat hematopoietic disease, heart failure, liver failure, bone defects, and cartilage defects.…”

Spatial micro-variation of 3D hydrogel stiffness regulates the biomechanical properties of hMSCs

“…Importantly, the biomechanical properties of cells are significantly regulated by the interplay between them and the extracellular matrix (ECM) [20,21]. The ECM regulates cellular behaviors via stimulus-responsive systems [22,23]; the cells identify the effective stimulus from the surrounding physical and chemical environment via transmembrane specific receptors, and the signal is transduced, resulting in the alteration of cellular phenotypes [24]. Of note, the collective performance of cells in 3D multicellular culture systems is attributed not only to the behavior of single cells but also to cell-cell and cell-substrate interactions.…”