Stem cells sense and respond to the mechanical properties of the extracellular matrix. However, both the extent to which extracellular matrix mechanics affect stem cell fate in 3D micro-environments and the underlying biophysical mechanisms are unclear. We demonstrate that the commitment of mesenchymal stem cell (MSC) populations changes in response to the rigidity of 3D micro-environments, with osteogenesis occurring predominantly at 11–30 kPa. In contrast to previous 2D work, however, cell fate was not correlated with morphology. Instead, matrix stiffness regulated integrin binding as well as reorganization of adhesion ligands on the nanoscale, both of which were traction-dependent and correlated with osteogenic commitment of MSC populations. These findings suggest that cells interpret changes in the physical properties of adhesion substrates as changes in adhesion ligand presentation, and that cells themselves can be harnessed as tools to mechanically process materials into structures that feedback to manipulate their fate.

Regenerative efforts typically focus on the delivery of single factors, but it is likely that multiple factors regulating distinct aspects of the regenerative process (e.g., vascularization and stem cell activation) can be used in parallel to affect regeneration of functional tissues. This possibility was addressed in the context of ischemic muscle injury, which typically leads to necrosis and loss of tissue and function. The role of sustained delivery, via injectable gel, of a combination of VEGF to promote angiogenesis and insulin-like growth factor-1 (IGF1) to directly promote muscle regeneration and the return of muscle function in ischemic rodent hindlimbs was investigated. Sustained VEGF delivery alone led to neoangiogenesis in ischemic limbs, with complete return of tissue perfusion to normal levels by 3 weeks, as well as protection from hypoxia and tissue necrosis, leading to an improvement in muscle contractility. Sustained IGF1 delivery alone was found to enhance muscle fiber regeneration and protected cells from apoptosis. However, the combined delivery of VEGF and IGF1 led to parallel angiogenesis, reinnervation, and myogenesis; as satellite cell activation and proliferation was stimulated, cells were protected from apoptosis, the inflammatory response was muted, and highly functional muscle tissue was formed. In contrast, bolus delivery of factors did not have any benefit in terms of neoangiogenesis and perfusion and had minimal effect on muscle regeneration. These results support the utility of simultaneously targeting distinct aspects of the regenerative process.

Targeting of nanoparticles to ischemic tissues was studied in a murine ischemic hindlimb model. Intravenously injected fluorescent nanoparticles allowed ischemia-targeted imaging of ischemic muscles due to increased permeability of blood vessels in hypoxic tissues. Targeting efficiency correlated with blood perfusion after induction of ischemia and was enhanced in early stage of ischemia (<7 days). Therapeutic delivery of vascular endothelial growth factor (VEGF) was achieved by VEGF-conjugated nanoparticles and resulted in a 1.7-fold increase in blood perfusion, as compared to control mice. This work supports the application of nanoparticles as imaging and therapeutic modalities for ischemia treatment.

SUMMARY The pancreatic islets of Langerhans regulate glucose homeostasis. The loss of insulin-producing β cells within islets results in diabetes, and islet transplantation from cadaveric donors can cure the disease. In vitro production of whole islets, not just β cells, will benefit from a better understanding of endocrine differentiation and islet morphogenesis. We used single-cell mRNA sequencing to obtain a detailed description of pancreatic islet development. Contrary to the prevailing dogma, we find islet morphology and endocrine differentiation to be directly related. As endocrine progenitors differentiate, they migrate in cohesion and form bud-like islet precursors, or “peninsulas” (literally “almost islands”). α cells, the first to develop, constitute the peninsular outer layer, and β cells form later, beneath them. This spatiotemporal collinearity leads to the typical core-mantle architecture of the mature, spherical islet. Finally, we induce peninsula-like structures in differentiating human embryonic stem cells, laying the ground for the generation of entire islets in vitro.