The extracellular matrix (ECM) is the primary biomechanical environment that interacts with tendon cells (tenocytes). Stresses applied via muscle contraction during skeletal movement transfer across structural hierarchies to the tenocyte nucleus in native uninjured tendons. Alterations to ECM structural and mechanical properties due to mechanical loading and tissue healing may affect this multiscale strain transfer and stress transmission through the ECM. This study explores the interface between dynamic loading and tendon healing across multiple length scales using living tendon explants. Results show that macroscale mechanical and structural properties are inferior following high magnitude dynamic loading (fatigue) in uninjured living tendon and that these effects propagate to the microscale. Although similar macroscale mechanical effects of dynamic loading are present in healing tendon compared to uninjured tendon, the microscale properties differed greatly during early healing. Regression analysis identified several variables (collagen and nuclear disorganization, cellularity, and F-actin) that directly predict nuclear deformation under loading. Finite element modeling predicted deficits in ECM stress transmission following fatigue loading and during healing. Together, this work identifies the multiscale response of tendon to dynamic loading and healing, and provides new insight into microenvironmental features that tenocytes may experience following injury and after cell delivery therapies.

Objective Collateral arteriogenesis, the growth of existing arterial vessels to a larger diameter, is a fundamental adaptive response that is often critical for the perfusion and survival of tissues downstream of chronic arterial occlusion(s). Shear stress regulates arteriogenesis; however, the arteriogenic significance of flow direction reversal, occurring in numerous collateral artery segments after femoral artery ligation (FAL), is unknown. Our objective was to determine if flow direction reversal in collateral artery segments differentially regulates endothelial cell signaling and arteriogenesis. Approach and Results Collateral segments experiencing flow reversal after FAL in C57BL/6 mice exhibit increased pericollateral macrophage recruitment, amplified arteriogenesis (30% diameter and 2.8-fold conductance increases), and remarkably permanent (12 weeks post-FAL) remodeling. Genome-wide transcriptional analyses on HUVECs exposed to flow reversal conditions mimicking those occurring in-vivo yielded 10-fold more significantly regulated transcripts, as well as enhanced activation of upstream regulators (NFκB, VEGF, FGF2, TGFβ) and arteriogenic canonical pathways (PKA, PDE, MAPK). Augmented expression of key pro-arteriogenic molecules (KLF2, ICAM-1, eNOS) was also verified by qRT-PCR, leading us to test whether ICAM-1 and/or eNOS regulate amplified arteriogenesis in flow-reversed collateral segments in-vivo. Interestingly, enhanced pericollateral macrophage recruitment and amplified arteriogenesis was attenuated in flow-reversed collateral segments after FAL in ICAM-1−/− mice; however, eNOS−/− mice showed no such differences. Conclusions Flow reversal leads to a broad amplification of pro-arteriogenic endothelial signaling and a sustained ICAM-1-dependent augmentation of arteriogenesis. Further investigation of the endothelial mechanotransduction pathways activated by flow reversal may lead to more effective and durable therapeutic options for arterial occlusive diseases.

At the plasma membrane interface, cells use various adhesions to sense their extracellular environment. These adhesions facilitate the transmission of mechanical signals that dictate cell behavior. This review discusses the mechanisms by which these mechanical signals are transduced through cell–matrix and cell–cell adhesions and how this mechanotransduction influences cell processes. Cell–matrix adhesions require the activation of and communication between various transmembrane protein complexes such as integrins. These links at the plasma membrane affect how a cell senses and responds to its matrix environment. Cells also communicate with each other through cell–cell adhesions, which further regulate cell behavior on a single- and multicellular scale. Coordination and competition between cell–cell and cell–matrix adhesions in multicellular aggregates can, to a significant extent, be modeled by differential adhesion analyses between the different interfaces even without knowing the details of cellular signaling. In addition, cell–matrix and cell–cell adhesions are connected by an intracellular cytoskeletal network that allows for direct communication between these distinct adhesions and activation of specific signaling pathways. Other membrane-embedded protein complexes, such as growth factor receptors and ion channels, play additional roles in mechanotransduction. Overall, these mechanoactive elements show the dynamic interplay between the cell, its matrix, and neighboring cells and how these relationships affect cellular function.

Arteriogenesis, or the lumenal expansion of pre-existing arterioles in the presence of an upstream occlusion, is a fundamental vascular growth response. Though alterations in shear stress stimulate arteriogenesis, the migration of monocytes into the perivascular space surrounding collateral arteries and their differentiation into macrophages is critical for this vascular growth response to occur. Focal adhesion kinase’s (FAK) role in regulating cell migration has recently been expanded to primary macrophages. We therefore investigated the effect of the myeloid-specific conditional deletion of FAK on vascular remodeling in the mouse femoral arterial ligation (FAL) model. Using laser Doppler perfusion imaging, whole mount imaging of vascular casted gracilis muscles, and immunostaining for CD31 in gastrocnemius muscles cross-sections, we found that there were no statistical differences in perfusion recovery, arteriogenesis, or angiogenesis 28 days after FAL. We therefore sought to determine FAK expression in different myeloid cell populations. We found that FAK is expressed at equally low levels in Ly6Chi and Ly6Clo blood monocytes, however expression is increased over 2-fold in bone marrow derived macrophages. Ultimately, these results suggest that FAK is not required for monocyte migration to the perivascular space and that vascular remodeling following arterial occlusion occurs independently of myeloid specific FAK.