肌腱是一種緊密對齊的結締組織，它兩端中一側與肌肉連接，並將其收縮的力量傳導至位於另一側的骨骼，因此肌腱負責帶動身體執行各種動作。但若反覆施力不當，有很高機率導致肌腱受傷。受傷類型包含急性撕裂傷和慢性結構變性。由於肌腱少細胞及少血管的特性，肌腱的自我修復效果低下且進程緩慢。為了重建完整的肌腱功能，幹細胞的應用正被積極研究中。目前這項備受期待的治療策略仍在面對容易引發鈣化的瓶頸，但若預先控制幹細胞的分化走向已知可有效減少此隱憂。根據先前的實驗結果顯示，細胞形狀的拉長能夠促使肌腱前驅幹細胞分化成肌腱細胞，但其機制還尚未釐清。由形狀調控的分化過程裡，細胞骨架肌動凝蛋白的收縮是必要的，因此讓我們聯想到受同樣機制調控的一對轉錄共活化蛋白Yes-associated protein (YAP)和transcriptional coactivator with PDZ-binding motif (TAZ)，他們可以穿梭細胞核膜，並影響細胞的DNA層面。例如，當細胞延展或接觸較硬的基質時，會促使YAP/TAZ入核，並且參與間質幹細胞的骨分化。另外，已知生長因子TGF-β3會促進肌腱分化，而YAP/TAZ具有其下游的Smad蛋白接合位，顯示出YAP/TAZ與肌腱分化的可能的關聯。因此，我們想要知道YAP/TAZ是否會參與由形狀調控以及由TGF-β3誘導的肌腱分化。在本研究中發現，肌腱前驅幹細胞不管是被塑成長方形或者被投與TGF-β3，其TAZ的變化相似於代表肌腱分化的蛋白SCX和TNMD的表現。此外，在TGF-β3誘導的肌腱分化中，使用YAP/TAZ抑制劑Verteporfin後，SCX和TNMD的表現也隨之下降，然而以微小干擾RNA調降YAP後卻無此現象。這些實驗結果暗示了TAZ可能參與拉長細胞形狀以及TGF-β3誘導的肌腱分化。我們期望未來對於肌腱分化機制的能有進一步瞭解，以促進幹細胞療法在未來臨床的應用。

Tendon is a well-aligned connective tissue which conveys muscle contraction to bone, and coordinates body movement. As a result, with repetitive tensile force, tendon often encounters injuries such as acute rupture and chronic tendinopathy. Owing to its acellular and avascular nature, tendon self-repair is slow and often incomplete. To reconstruct functional tendon, stem cell therapy is one of developing strategies. However, the problem of ectopic bone formation occurs after stem cells implant. Recently, some have found that leading the stem cell differentiation toward tendon lineage in advance can remove the obstacle effectively. In our lab, by elongating the cell shape, we have successfully induced tenogenesis on tendon stem/ progenitor cells (TSPCs). We were interested in delineating the molecular mechanisms underlying tendon differentiation induced by microenvironmental cues using geometrical control. Given that actomyosin contractility is pivotal for the tendon differentiated process, we then focused on Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). YAP and TAZ are transcriptional coactivators, known as mechanotransducers, shuttling into the nucleus and elicit cell response at DNA level indirectly. The nuclear translocation of YAP and TAZ regulate osteogenic differentiation in mesenchymal stem cells while cells were spreading or on stiff substrate. Moreover, YAP and TAZ have binding sites on Smad, the downstream proteins of canonical transforming growth factor-beta (TGF-β) signal cascade. TGF-βs are essential for tenogenesis, and the fact implies the potential function of YAP/TAZ in tenogenesis. Therefore, we hypothesized that YAP/TAZ mediated shape-controlled and TGF-β-induced tenogenesis. Using rectangle micropattern and TGF-β3 as main inducers, we found that TSPCs possessed a positive relationship between TAZ and the tendon specific markers, such as scleraxis (SCX) and tenomodulin (TNMD). In addition, knockdown YAP by siRNA did not affect TGF-β3-induced tenogenic differentiation whereas YAP/TAZ inhibitor Verteporfin (VP) resulted in a decrease of SCX and TNMD. These results suggest that TAZ plays a role in tenogenesis. Understanding of the molecular mechanisms of how environmental cues regulate tenogenic differentiation would help us for establish better strategies for the treatment of tendon injury.

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