Increased Ca ²⁺ signaling through Ca _V 1.2 induces tendon hypertrophy with increased collagen fibrillogenesis and biomechanical properties

Tendons are tension-bearing tissues transmitting force from muscle to bone for body movement. This mechanical loading is essential for tendon development, homeostasis, and healing after injury. While Ca²⁺ signaling has been studied extensively for its roles in mechanotransduction, regulating muscle, bone, and cartilage development and homeostasis, knowledge about Ca²⁺ signaling and the source of Ca²⁺ signals in tendon fibroblast biology are largely unknown. Here, we investigated the function of Ca²⁺ signaling through Ca_V 1.2 voltage-gated Ca²⁺ channel in tendon formation. Using a reporter mouse, we found that Ca_V 1.2 is highly expressed in tendon during development and downregulated in adult homeostasis. To assess its function, we generated ScxCre;Ca_V 1.2^TS mice that express a gain-of-function mutant Ca_V 1.2 in tendon. We found that mutant tendons were hypertrophic, with more tendon fibroblasts but decreased cell density. TEM analyses demonstrated increased collagen fibrillogenesis in the hypertrophic tendons. Biomechanical testing revealed that the hypertrophic tendons display higher peak load and stiffness, with no changes in peak stress and elastic modulus. Proteomic analysis showed no significant difference in the abundance of type I and III collagens, but mutant tendons had about two-fold increase in other ECM proteins such as tenascin C, tenomodulin, periostin, type XIV and type VIII collagens, around 11-fold increase in the growth factor myostatin, and significant elevation of matrix remodeling proteins including Mmp14, Mmp2, and cathepsin K. Taken together, these data highlight roles for increased Ca²⁺ signaling through Ca_V 1.2 on regulating expression of myostatin growth factor and ECM proteins for tendon collagen fibrillogenesis during tendon formation.