Pericellular Proteolysis and the Regulation of Bone/Tendon Stem Cell Fate

摘要 骨骼和肌腱都含有嵌入在富含I型胶原的组织中的干细胞群。骨 骨髓和骨内膜来源的干细胞通过软骨内成骨生成骨，而骨膜干细胞通过软骨内成骨生成骨。 细胞通过膜内途径形成骨。反过来，肌腱干细胞产生肌腱细胞， 成熟的组织最近的研究已经确定半胱氨酸蛋白酶，组织蛋白酶K（CTSK），作为一种独特的， 我们已经证实了这一发现，但这也导致我们确定了迄今为止 与肌腱相关的未表征的第二股骨柄群体。在骨髓来源的MSC中，我们有 先前确定了一种新的需要膜锚定金属蛋白酶，MT 1-MMP， 调节机械敏感性、雅普/TAZ依赖性途径，该途径通过以下途径控制干细胞谱系定型： 细胞周围胶原基质的蛋白水解重塑。相比之下，MT 1-MMP和 骨膜或肌腱/韧带干细胞中的CTSK是未知的。使用Ctsk-Cre转基因系靶向 骨膜干细胞，我们发现，Mt 1-MMP靶向单独elevens一个深刻的骨质减少状态，在体内， 不仅独立于Ctsk运作，而且还将干细胞重定向到异常的过度增殖， 软骨形成状态此外，出乎意料的是，Ctsk-Cre依赖性靶向MT 1-MMP -但不是Ctsk， 通过改变以前未描述的腱旁干细胞-腱运输途径破坏腱/结构。在 在体内，MT 1-MMP-null肌腱干细胞致力于过度增殖，软骨形成表型类似于 在肌腱/韧带创伤后观察到。基于这些新的数据，我们提出MT 1-MMP控制 这些干细胞群中的每一个都通过调节机械转导途径， 雅普/TAZ连接的共转录程序，而且也是通过蛋白水解的典型TFGβ信号通路。 辅助TGFβ受体TGFβRIII脱落。总之，这些发现概述了MSC的新模型 其中MT 1-MMP依赖性胶原溶解和受体脱落共同发挥所需的作用， 骨膜组织和肌腱。因此，我们提出了3个目标：i）表征MT 1-MMP在调节 骨膜干细胞的分化和功能在体外和体内，ii）定义了一个新的作用，Ctsk+肌腱干 细胞在控制肌腱功能及其调节MT 1-MMP和iii）描绘的影响MT 1-MMP对 骨膜/腱旁干细胞雅普/TAZ力学转导和TGFβ/TGFβRIII依赖性分化 程序.总之，这些目标应该为需要MT 1的特化干细胞家族带来新的曙光。 MMP依赖的蛋白水解不仅调节膜内骨形成，还调节肌腱 结构/功能。

ABSTRACT Bone as well as tendons each contain stem cell populations embedded in type I collagen-rich tissues. Bone marrow and endosteal-derived stem cells generate bone via endochondral ossification while periosteal stem cells form bone via the intramembranous route. In turn, tendon stem cells give rise to tenocytes that populate the mature tissues. Recent studies have identified the cysteine proteinase, cathepsin K (CTSK), as a unique marker of periosteal stem cells, a finding we have confirmed, but that also led to our identification of a heretofore uncharacterized second stem population associated with tendons. In bone marrow-derived MSCs, we have previously identified a novel requirement for the membrane-anchored metalloproteinase, MT1-MMP, in regulating a mechanosensitive, YAP/TAZ-dependent pathway that controls stem cell lineage commitment via the proteolytic remodeling of the pericellular collagen matrix. By contrast, the relative roles of MT1-MMP and CTSK in periosteal or tendon/ligament stem cells are unknown. Using a Ctsk-Cre transgenic line to target periosteal stem cells, we find that Mt1-mmp targeting alone elicits a profound osteopenic state in vivo that not only operates independently of Ctsk, but also redirects the stem cells to an aberrant hyperproliferative, chondrogenic state. Further, and unexpectedly, Ctsk-Cre–dependent targeting of MT1-MMP - but not Ctsk, disrupts tendon/ structure by altering a previously undescribed paratenon stem cell-tendon trafficking route. In vivo, MT1-MMP-null tendon stem cells commit to a hyperproliferative, chondrogenic phenotype similar to that observed following tendon/ligament trauma. Based on these new data, we propose that MT1-MMP controls each of these stem cell populations by regulating a mechanotransduction pathway that not only controls YAP/TAZ-linked co-transcriptional programs, but also canonical TFGβ signaling pathways via the proteolytic shedding of the accessory TGFβ receptor, TGFβRIII. Together, these findings outline a new model of MSC function wherein MT1-MMP-dependent collagenolysis and receptor shedding together play a required role in periosteal tissues and tendons. As such, we propose 3 aims; i) characterize the role of MT1-MMP in regulating periosteal stem cell differentiation and function in vitro and in vivo, ii) define a novel role for Ctsk+ tendon stem cells in controlling tendon function and its regulation by MT1-MMP and iii) delineate the impact of MT1-MMP on periosteal/paratenon stem cell YAP/TAZ mechanotransduction and TGFβ/TGFβRIII- dependent differentiation programs. Together, these aims should cast new light on a family of specialized stem cells that require MT1- MMP-dependent proteolysis to not only regulate intramembranous bone formation, but also tendon structure/function as well.