Mechanotransductive feedback control of endothelial cell motility and vascular morphogenesis

Vascular morphogenesis requires persistent endothelial cell motility that is responsive to diverse and dynamic mechanical stimuli. Here, we interrogated the mechanotransductive feedback dynamics that govern endothelial cell motility and vascular morphogenesis. We show that the transcriptional regulators, YAP and TAZ, are activated by mechanical cues to transcriptionally limit cytoskeletal and focal adhesion maturation, forming a conserved mechanotransductive feedback loop that mediates human endothelial cell motility in vitro and zebrafish intersegmental vessel (ISV) morphogenesis in vivo. This feedback loop closes in 4 hours, achieving cytoskeletal equilibrium in 8 hours. Feedback loop inhibition arrested endothelial cell migration in vitro and ISV morphogenesis in vivo. Inhibitor washout at 3 hrs, prior to feedback loop closure, restored vessel growth, but washout at 8 hours, longer than the feedback timescale, did not, establishing lower and upper bounds for feedback kinetics in vivo. Mechanistically, YAP and TAZ induced transcriptional suppression of myosin II activity to maintain dynamic cytoskeletal equilibria. Together, these data establish the mechanoresponsive dynamics of a transcriptional feedback loop necessary for persistent endothelial cell migration and vascular morphogenesis

YAP and TAZ Mediate Osteocyte Perilacunar/Canalicular Remodeling

Bone fragility fractures are caused by low bone mass or impaired bone quality. Osteoblast/osteoclast coordination determines bone mass, but the factors that control bone quality are poorly understood. Osteocytes regulate osteoblast and osteoclast activity on bone surfaces but can also directly reorganize the bone matrix to improve bone quality through perilacunar/canalicular remodeling; however, the molecular mechanisms remain unclear. We previously found that deleting the transcriptional regulators Yes-associated protein (YAP) and Transcriptional co-activator with PDZ-motif (TAZ) from osteoblast-lineage cells caused lethality in mice due to skeletal fragility. Here, we tested the hypothesis that YAP and TAZ regulate osteocyte-mediated bone remodeling by conditional ablation of both YAP and TAZ from mouse osteocytes using 8kb-DMP1-Cre. Osteocyte-conditional YAP/TAZ deletion reduced bone mass and dysregulated matrix collagen content and organization, which together decreased bone mechanical properties. Further, YAP/TAZ deletion impaired osteocyte perilacunar/canalicular remodeling by reducing canalicular network density, length, and branching, as well as perilacunar flourochrome-labeled mineral deposition. Consistent with recent studies identifying TGF-β as a key inducer of osteocyte expression of matrix-remodeling enzymes, YAP/TAZ deletion in vivo decreased osteocyte expression of matrix proteases MMP13, MMP14, and CTSK. In vitro, pharmacologic inhibition of YAP/TAZ transcriptional activity in osteocyte-like cells abrogated TGF-β-induced matrix protease gene expression. Together, these data show that YAP and TAZ control bone matrix accrual, organization, and mechanical properties by regulating osteocyte-mediated bone remodeling. Elucidating the signaling pathways that control perilacunar/canalicular remodeling may enable future therapeutic targeting of bone quality to reverse skeletal fragility

Mechanical regulation of bone regeneration and vascular growth in vivo

Regeneration of large bone defects presents a critical challenge to orthopaedic clinicians as the current treatment strategies are severely limited. Tissue engineering has therefore emerged as a promising alternative to bone grafting techniques. This approach features the delivery of bioactive agents such as stem cells, genes, or proteins using biomaterial delivery systems which together stimulate endogenous repair mechanisms to regenerate the tissue. Because bone is a highly mechanosensitive tissue which responds and adapts dynamically to its mechanical environment, application of mechanical stimuli may enhance endogenous tissue repair. While mechanical loading has been shown to stimulate bone fracture healing, the ability of loading to enhance large bone defect regeneration has not been evaluated. The goal of this thesis was to evaluate the ability of sustained osteogenic growth factor delivery and functional biomechanical loading to stimulate vascularized repair of large bone defects in a rat segmental defect model. First, we evaluated the hypothesis that the relationship between protein dose and regenerative efficacy depends on delivery system. We determined the dose-response relationship between dose of recombinant human bone morphogenetic protein-2 (rhBMP-2) and bone regeneration in a hybrid alginate-based protein delivery system and compared with the current clinically-used collagen sponge. The hybrid delivery system improved bone formation and reduced the effective dose due to its sustained delivery properties in vivo. Next, we tested the hypothesis that transfer of compressive ambulatory loads during segmental defect repair enhances bone formation and subsequent limb regeneration. We found that delayed application of axial loads enhanced bone regeneration by altering bone formation, tissue differentiation and remodeling, and local strain distribution. Finally, we evaluated the hypothesis that in vivo mechanical loading can enhance neovascular growth to influence bone formation. We found that early mechanical loading disrupted neovascular growth, resulting in impaired bone healing, while delayed loading induced vascular remodeling and enhanced bone formation. Together, this thesis presents the effects of dose and delivery system on BMP-mediated bone regeneration and demonstrates for the first time the effects of in vivo mechanical loading on vascularized regeneration of large bone defects.Ph.D.Committee Chair: Guldberg, Robert; Committee Member: Garcia, Andres; Committee Member: Gleason, Rudolph; Committee Member: Taylor, W. Robert; Committee Member: Zamir, Eva

Heterogeneity of Cellular Hypoxia in Murine Bone Marrow

<p>This abstract (Paper No. 482) was presented at the Annual Meeting of the Orthopedic Research Society (ORS) - February 10 – 14, 2023, Dallas, Texas, USA.</p><p>The abstract has been published previously online here https://www.ors.org/transactions/2023/482.pdf</p&gt

BMP-SMAD1/5 Signaling Is Required For Adequate Coupling Of Angiogenesis And Osteogenesis In Long Bones

The bone vasculature is essential for skeletal development, homeostasis, and regeneration. In long bones, specific capillary subtypes couple angiogenesis to osteogenesis, especially at the growth plate (metaphysis) and endosteum. These metaphyseal and endosteal vessels, termed “type H,” express high levels of CD31 and endomucin (CD31hiEmcnhi), show a columnar structure and are closely related to Osterix (Osx)- and Runt-related transcription factor 2 (Runx2)-expressing osteoblasts and their progenitors [1]. Sinusoidal vessels with low expression of CD31 and Emcn (CD31lowEmcnlow, type L vessels) are mainly found in the diaphysis/bone marrow cavity [1]. The crosstalk between endothelial cells (ECs) of type H vessels and bone-related cells is characterized by osteogenesis-promoting factors provided by ECs and angiogenic mediators secreted by osteoblast-lineage cells. Bone morphogenetic proteins (BMP) are major regulators of vessel formation, however their role within angiogenic and osteogenic coupling and in particular, the involvement of BMP-related intracellular effectors SMAD1 and SMAD5 (SMAD1/5) is unclear. Previously, we demonstrated that during mouse embryonic development, the synergy of Notch and SMAD1/5 is responsible for the balanced selection of tip and stalk cells in vascular sprouting [2]. Furthermore, we reported that endothelium-specific deletion of SMAD1/5 during early postnatal retinal angiogenesis resulted in arteriovenous malformations, a reduced number of tip cells and a hyperdensity in the vascular plexus [3]. Therefore, we hypothesized that functional EC-specific SMAD1/5 signaling is required for (i) formation and maturation of long bone vasculature and (ii) coupling of osteogenesis and angiogenesis, specifically of type H vessel formation in the metaphysis

Mechanoregulation of MSC spheroid immunomodulation

Mesenchymal stromal cells (MSCs) are widely used in cell-based therapies and tissue regeneration for their potent secretome, which promotes host cell recruitment and modulates inflammation. Compared to monodisperse cells, MSC spheroids exhibit improved viability and increased secretion of immunomodulatory cytokines. While mechanical stimulation of monodisperse cells can increase cytokine production, the influence of mechanical loading on MSC spheroids is unknown. Here, we evaluated the effect of controlled, uniaxial cyclic compression on the secretion of immunomodulatory cytokines by human MSC spheroids and tested the influence of load-induced gene expression on MSC mechanoresponsiveness. We exposed MSC spheroids, entrapped in alginate hydrogels, to three cyclic compressive regimes with varying stress (L) magnitudes (i.e., 5 and 10 kPa) and hold (H) durations (i.e., 30 and 250 s) L5H30, L10H30, and L10H250. We observed changes in cytokine and chemokine expression dependent on the loading regime, where higher stress regimes tended to result in more exaggerated changes. However, only MSC spheroids exposed to L10H30 induced human THP-1 macrophage polarization toward an M2 phenotype compared to static conditions. Static and L10H30 loading facilitated a strong, interlinked F-actin arrangement, while L5H30 and L10H250 disrupted the structure of actin filaments. This was further examined when the actin cytoskeleton was disrupted via Y-27632. We observed downregulation of YAP-related genes, and the levels of secreted inflammatory cytokines were globally decreased. These findings emphasize the essential role of mechanosignaling in mediating the immunomodulatory potential of MSC spheroids

An alginate-based hybrid system for growth factor delivery in the functional repair of large bone defects

The treatment of challenging fractures and large osseous defects presents a formidable problem for orthopaedic surgeons. Tissue engineering/regenerative medicine approaches seek to solve this problem by delivering osteogenic signals within scaffolding biomaterials. In this study, we introduce a hybrid growth factor delivery system that consists of an electrospun nanofiber mesh tube for guiding bone regeneration combined with peptide-modified alginate hydrogel injected inside the tube for sustained growth factor release. We tested the ability of this system to deliver recombinant bone morphogenetic protein-2 (rhBMP-2) for the repair of critically-sized segmental bone defects in a rat model. Longitudinal [mu]-CT analysis and torsional testing provided quantitative assessment of bone regeneration. Our results indicate that the hybrid delivery system resulted in consistent bony bridging of the challenging bone defects. However, in the absence of rhBMP-2, the use of nanofiber mesh tube and alginate did not result in substantial bone formation. Perforations in the nanofiber mesh accelerated the rhBMP-2 mediated bone repair, and resulted in functional restoration of the regenerated bone. [mu]-CT based angiography indicated that perforations did not significantly affect the revascularization of defects, suggesting that some other interaction with the tissue surrounding the defect such as improved infiltration of osteoprogenitor cells contributed to the observed differences in repair. Overall, our results indicate that the hybrid alginate/nanofiber mesh system is a promising growth factor delivery strategy for the repair of challenging bone injuries

Endothelial SMAD1/5 signaling couples angiogenesis to osteogenesis in juvenile bone

Abstract Skeletal development depends on coordinated angiogenesis and osteogenesis. Bone morphogenetic proteins direct bone formation in part by activating SMAD1/5 signaling in osteoblasts. However, the role of SMAD1/5 in skeletal endothelium is unknown. Here, we found that endothelial cell-conditional SMAD1/5 depletion in juvenile mice caused metaphyseal and diaphyseal hypervascularity, resulting in altered trabecular and cortical bone formation. SMAD1/5 depletion induced excessive sprouting and disrupting the morphology of the metaphyseal vessels, with impaired anastomotic loop formation at the chondro-osseous junction. Endothelial SMAD1/5 depletion impaired growth plate resorption and, upon long-term depletion, abrogated osteoprogenitor recruitment to the primary spongiosa. Finally, in the diaphysis, endothelial SMAD1/5 activity was necessary to maintain the sinusoidal phenotype, with SMAD1/5 depletion inducing formation of large vascular loops and elevated vascular permeability. Together, endothelial SMAD1/5 activity sustains skeletal vascular morphogenesis and function and coordinates growth plate remodeling and osteoprogenitor recruitment dynamics in juvenile mouse bone