Mechanical loading is required for initiation of extracellular matrix deposition at the developing murine myotendinous junction

Lack of skeletal muscle contraction disrupts fibrous tissue morphogenesis in the developing murine knee

“…Earlier time points (e.g., TS22/E13.5) are also needed to determine how the initial formation of these tissues might be impacted in the absence of muscle loading, which would elucidate the importance of muscle loading compared to other biophysical (e.g., passive tension) or biochemical cues. 44 Taken together, our results demonstrate that muscle contraction is critical for the continued growth and maintenance of all dense connective tissues in the knee joint. Why muscle contraction has a more prominent role in certain tissues, and regions within tissues, F I G U R E 6 Impact of immobilization on knee joint fibrous tissue morphogenesis.…”

“…However, we are currently expanding this line in our colony again and future studies will investigate the specific biomolecular mechanisms that led to the observed phenotypes, including additional time points and analyses (e.g., gene expression, immunofluorescence for matrix composition, apoptosis staining, and catabolic enzyme activity assays). Earlier time points (e.g., TS22/E13.5) are also needed to determine how the initial formation of these tissues might be impacted in the absence of muscle loading, which would elucidate the importance of muscle loading compared to other biophysical (e.g., passive tension) or biochemical cues 44 …”

Fibroblast diversification is an embryonic process dependent on muscle contraction

“…Concomitant with the emergence of Ly6a + fibroblasts is the transition from embryonic to fetal myogenesis at around E14.5. Remarkably, this transition is accompanied by maturation of the muscle contractile units, MTJ formation, and initiation of myofiber contractions, which are known to be crucial for limb patterning and muscle transcriptional changes during development (Dos Santos et al, 2023; Felsenthal & Zelzer, 2017; Huang et al, 2015; Lipp et al, 2023). Therefore, to test whether muscle contraction could impact fibroblasts diversification we analyzed limbs of muscle dysgenic ( mdg ) embryos, harboring a lethal point mutation in the skeletal muscle specific Cacna1s gene which encodes the alpha subunit of Cav1.1 calcium channel, a lethal mutation resulting in paralyzed non-contractile skeletal muscles.…”

“…Our results indicate fibroblasts to have started diversifying by E15.5, accompanying the initiation of muscle contraction following the transition from primary to secondary myogenesis. Muscle contraction has been reported to be crucial for the development of several components of the musculoskeletal system in the embryo, including bone morphogenesis (Felsenthal & Zelzer, 2017), tendon patterning (Huang et al, 2015), MTJ and myofiber maturation (Dos Santos et al, 2023; Lipp et al, 2023). Results from the paralyzed mdg mutant showed loss of expression of Ly6a+ fibroblasts markers in the muscle periphery, suggesting that muscle contraction initiates these fibroblasts’ diversification.…”

Extracellular matrix protein composition dynamically changes during murine forelimb development

“…Recently, liquid chromatography-tandem mass spectrometry (LC-MS/MS) and tissue fractionation were combined to identify how the composition of ECM proteins varies between different adult and pathological tissues (reviewed in Taha et al., and McCabe et al., 17 , 18 ). Our lab extended this approach to analyze the matrisome of embryonic murine tissues 19 , 20 , 21 , 22 ; however, this only provided a snapshot of the static matrisome at certain stages of development and was unable to resolve when specific proteins were made. Identification of the proteins synthesized at a given time point will provide additional information about the dynamics of critical components that drive changes in tissue structure and remodeling during development.…”