Physiological load via voluntary wheel running maintains Achilles tendon homeostasis by region-specific cellular responses
Physiological load is vital for maintaining tendon homeostasis, preserving the organized extracellular matrix that enables tendons to withstand extreme forces. Although tenocytes are regarded as the primary regulators of extracellular matrix production, precisely how cells facilitate the maintenance of homeostasis in response to physiological load is poorly understood. Here, we used Voluntary Wheel Running (VWR) as a model of physiological load to delineate the specific cellular contributions to mouse Achilles tendon homeostasis. Eight weeks of VWR led to a smaller cross-sectional area, increased mechanical and material properties at the midsubstance, which corresponded to a decreased proportion of small (0-60 nm) collagen fibrils and an increased proportion of larger (100-60 nm) collagen fibrils compared to sedentary controls. Using Visium HD spatial transcriptomics, we identified region-specific cell clusters (insertion vs. midsubstance). In response to physiological load, cells in the insertion and midsubstance upregulate distinct genes that reinforce the fibrocartilage interface and collagen-rich tendon matrix, respectively. Notably, Clu, Myoc, and Ccdc80 were upregulated with VWR in the midsubstance, with previously uncharacterized roles in tendon homeostasis. Together, our findings suggest that in response to physiological load, tendon cells maintain homeostasis by region-specific responses. Given that insertional and midsubstance tendinopathy is function-limiting and painful, defining the region-specific cellular responses will be key to advancing therapeutic prospects for tendon health. New and NoteworthyThis study is the first spatially rigorous characterization of the tendon response to physiological load using a Voluntary Wheel Running (VWR) model. VWR led to smaller, stronger, but not stiffer tendons at the midsubstance compared to sedentary controls. This corresponded with significant decreased proportion of small collagen fibrils and a shift toward an increased proportion of large collagen fibrils. Using Visium HD spatial transcriptomics, we identified region-specific transcriptional responses to physiological load that maintain homeostasis. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=67 SRC="FIGDIR/small/722285v1_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@13aa717org.highwire.dtl.DTLVardef@1afe3f1org.highwire.dtl.DTLVardef@9e1137org.highwire.dtl.DTLVardef@17c47bc_HPS_FORMAT_FIGEXP M_FIG C_FIG