Integrin-Adhesion Ligand Bonds as 3D Mechanosensors that Modulate Mesenchymal Stem Cell Fate

Cell-based therapies have exciting clinical promise. In particular, bone marrow derived mesenchymal stem cells (MSC) can be easily isolated and have the potential to repair a variety of tissues. However, key challenges, including substantial loss of viability, impair cell therapy efforts. Biomaterials may augment cell therapies by providing a substrate that improves cell survival, while simultaneously locally manipulating cell fate. The hypothesis driving this work is that physical cues from these materials, including elasticity and the formation and distribution of micron-scale pores, can be manipulated to influence MSC. MSC responses to integrin-binding peptides were first studied with cell-encapsulating, RGD-modified alginate hydrogels. In these studies, a high density of RGD peptides, in concert with soluble factors, promoted osteogenic (bone) differentiation in vitro. Next, the potential that MSC fate can be controlled by manipulating the elasticity of 3D hydrogels was studied. In these studies, osteogenesis was predominant in materials with elastic modulus near 20 kPa. However, cell shape, previously identified as a putative "mechanosensor" from 2D studies, did not correlate with fate. It was hypothesized instead that nanoscale changes in the cell-matrix interface might underlie these fate changes. Using techniques based on Förster Resonance Energy Transfer (FRET) or newly developed biochemical methods, it was discovered that matrix rigidity controlled cells' ability to bind RGD peptides grafted to the hydrogel, and the range of elasticity which was optimal for osteogenesis was also optimal for αV and α5-integrin-RGD bond formation. Finally, a means to create macroscale pores within hydrogel materials, independent of the elasticity of the hydrogel surrounding pores, was developed. The composite materials formed using this technique were injectable, and pore formation appeared to be relatively independent of the hydrogel material surrounding micro-beads which eventually degraded in situ to form pores. Porogen degradation could be tuned to manipulate the kinetics of MSC deployment from these materials, and cell fate could further be altered by modulating hydrogel elasticity and RGD density. These studies, and the techniques developed and refined to perform them, should improve our basic understanding of design parameters for fabricating materials to influence cell fate

Spatiotemporal delivery of bone morphogenetic protein enhances functional repair of segmental bone defects

Osteogenic growth factors that promote endogenous repair mechanisms hold considerable potential for repairing challenging bone defects. The local delivery of one such growth factor, bone morphogenetic protein (BMP), has been successfully translated to clinical practice for spinal fusion and bone fractures. However, improvements are needed in the spatial and temporal control of BMP delivery to avoid the currently used supraphysiologic doses and the concomitant adverse effects. We have recently introduced a hybrid protein delivery system comprised of two parts: a perforated nanofibrous mesh that spatially confines the defect region and a functionalized alginate hydrogel that provides temporal growth factor release kinetics. Using this unique spatiotemporal delivery system, we previously demonstrated BMP-mediated functional restoration of challenging 8 mm femoral defects in a rat model. In this study, we compared the efficacy of the hybrid system in repairing segmental bone defects to that of the current clinical standard, collagen sponge, at the same dose of recombinant human BMP-2. In addition, we investigated the specific role of the nanofibrous mesh tube on bone regeneration. Our results indicate that the hybrid delivery system significantly increased bone regeneration and improved biomechanical function compared to collagen sponge delivery. Furthermore, we observed that presence of the nanofiber mesh tube was essential to promote maximal mineralized matrix synthesis, prevent extra-anatomical mineralization, and guide an integrated pattern of bone formation. Together, these results suggest that spatiotemporal strategies for osteogenic protein delivery may enhance clinical outcomes by improving localized protein retention

Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies

Flare frequency distributions represent a key approach to addressing one of the largest problems in solar and stellar physics: determining the mechanism that counter-intuitively heats coronae to temperatures that are orders of magnitude hotter than the corresponding photospheres. It is widely accepted that the magnetic field is responsible for the heating, but there are two competing mechanisms that could explain it: nanoflares or Alfv\'en waves. To date, neither can be directly observed. Nanoflares are, by definition, extremely small, but their aggregate energy release could represent a substantial heating mechanism, presuming they are sufficiently abundant. One way to test this presumption is via the flare frequency distribution, which describes how often flares of various energies occur. If the slope of the power law fitting the flare frequency distribution is above a critical threshold, $\alpha=2$ as established in prior literature, then there should be a sufficient abundance of nanoflares to explain coronal heating. We performed $>$600 case studies of solar flares, made possible by an unprecedented number of data analysts via three semesters of an undergraduate physics laboratory course. This allowed us to include two crucial, but nontrivial, analysis methods: pre-flare baseline subtraction and computation of the flare energy, which requires determining flare start and stop times. We aggregated the results of these analyses into a statistical study to determine that $\alpha = 1.63 \pm 0.03$. This is below the critical threshold, suggesting that Alfv\'en waves are an important driver of coronal heating.Comment: 1,002 authors, 14 pages, 4 figures, 3 tables, published by The Astrophysical Journal on 2023-05-09, volume 948, page 7