Potential of Hematopoietic Stem Cell-Based Therapies for Complicated Fractures

四肢肌肉骨骼损伤占OIF/OEF/OND退伍军人所有战斗创伤的约50%。爆炸 简易爆炸装置造成的伤害占战斗伤亡的75%以上，幸存者 经历了严重的骨科损伤这些类型的骨科损伤通常导致延迟愈合或不愈合， 其中正常的骨愈合受损。目前对不愈合骨折的干预措施，特别是萎缩性骨折 不愈合的骨折很少成功。OIF/OEF/OND退伍军人也有患抑郁症的高风险， 抑郁症相关的疾病，包括创伤后应激障碍（PTSD），导致> 60%的这类疾病 正在服用抗抑郁药的人群，特别是选择性5-羟色胺再摄取抑制剂（SSRIs）。而 SSRIs对骨折愈合的影响尚不清楚，已表明舍曲林（左洛复）和帕罗西汀 （Paxil），用于这些疾病的药物治疗的一线SSRIs，对骨骼健康产生负面影响。给定 复杂骨科损伤的患病率，这些损伤的长期并发症， 由于SSRI在OIF/OEF/OND退伍军人群体中使用的普遍性，需要新的疗法（例如， 基于细胞的疗法），其可以克服复杂骨折的当前临床限制。 我们的研究使用独特的克隆细胞移植模型结合小鼠骨折模型， 将造血干细胞（HSC）鉴定为成骨细胞、骨细胞的新型祖细胞， 软骨细胞在骨折修复中的作用最近的研究表明，HSC衍生的骨祖细胞直接引起 体内骨。这些发现是范式的转变，因为大多数研究都集中在间充质干细胞的使用上。 骨髓间充质干细胞（MSC）用于肌肉骨骼损伤修复，并表明基于HSC的治疗对复杂的 骨折基于我们之前的MERIT研究，该研究证明了骨形态发生蛋白-2的作用， BMP-2、BMP-9和胰岛素样生长因子（IGF-2）在促进HSC成骨中的作用，我们现在已经 显示IGF-2 + BMP-9的组合导致HSC成骨前体的骨诱导增强。 我们还鉴定了HSC衍生的循环成骨祖细胞（COP），其在造血干细胞移植过程中血液中增加。 正常的骨折修复和动员与AMD 3100交付，这表明一个潜在的治疗方式。 我们的初步数据还表明，SSRI管理在体内导致受损的骨愈合，结果 在愈合过程中改变成骨特征，并抑制体外HSC祖细胞的骨诱导。 我们推测HSC来源的骨软骨祖细胞可能是增强修复的靶细胞 复杂的骨折我们的目标是检测HSC来源的骨软骨祖细胞的能力， （与MSC相比）作为复杂性骨折不愈合和骨折的治疗方式 在SSRI给药期间，揭示这些细胞可能具有有益作用的机制， 阐明SSRIs对这种独特的祖细胞群体的影响。具体目标是确定1） 如果HSC衍生的骨软骨形成前体在体内的调节促进萎缩性骨关节炎模型中的愈合， 骨不连和2）如果SSRI通过抑制HSC衍生的成骨前体损害骨折愈合。 这些研究的意义在于，它们通过提出一种新的HSC起源来挑战现有的教条， 骨细胞和创新，因为他们将决定利用HSC的潜力，以加强愈合， 复杂骨折例。这项研究的结果很容易转化为临床（HSC动员是FDA 批准用于其他应用程序），并有可能直接影响退伍军人医疗保健。理解 SSRIs如何影响骨愈合和特定的干细胞隔室可能具有巨大的临床影响 因为有大量受伤的退伍军人开了SSRIs这项工作将有深远的利益， 军事人员和退伍军人复杂性骨折的高风险，特别是那些OIF/OEF/OND， 使用SSRI治疗临床抑郁症和/或PTSD，因此与VA使命高度相关。

Musculoskeletal extremity injuries comprise ~50% of all combat wounds for OIF/OEF/OND Veterans. Blast injuries via improvised explosive devices account for over 75% of combat casualties, with survivors experiencing severe orthopedic injuries. These types of orthopedic injuries often result in delayed or non-union, wherein normal bone healing is impaired. Current interventions for non-union fractures, particularly atrophic non-union fractures are rarely successful. OIF/OEF/OND Veterans are also at a high risk for depressive and depressive-associated disorders including post-traumatic stress disorder (PTSD), resulting in >60% of this population being prescribed antidepressants, specifically selective serotonin reuptake inhibitors (SSRIs). While the effects of SSRIs on fracture healing are unknown, it has been shown that Sertraline (Zoloft) and Paroxetine (Paxil), first-line SSRIs for pharmacological treatment of these disorders, negatively affect bone health. Given the prevalence of complex orthopedic injuries, the long-term complications from these injuries, and the commonness of SSRI use in the OIF/OEF/OND Veteran population, there is a need for new therapies (e.g., cell-based therapies) that may overcome current clinical limitations for complicated fractures. Our studies using a unique clonal cell transplantation model in conjunction with murine fracture models has identified the hematopoietic stem cell (HSC) as a novel progenitor for osteoblasts, osteocytes, and chondrocytes during fracture repair. Recent studies show HSC-derived osteoprogenitors directly give rise to bone in vivo. These findings are paradigm shifting in that most studies focus on the use of mesenchymal stem cells (MSCs) for musculoskeletal injury repair and suggest a benefit of HSC-based therapies for complicated fracture. Building on our previous MERIT studies, which demonstrated a role for bone morphogenetic protein-2 (BMP-2), BMP-9, and insulin-like growth factor (IGF-2) in promoting osteogenesis from the HSC, we have now shown a combination of IGF-2+BMP-9 results in enhanced osteoinduction from HSC-osteogenic precursors. We have also identified HSC-derived circulating osteogenic progenitors (COPs) that increase in blood during normal fracture repair and are mobilized with AMD3100 delivery, suggesting a potential therapeutic modality. Our preliminary data also demonstrate that SSRI administration in vivo leads to impaired bone healing, results in altered osteogenic profiles during healing, and inhibits osteoinduction from HSC-progenitors in vitro. We hypothesize that HSC-derived osteo-chondrogenic progenitor cells may be targeted to enhance repair of complicated fractures. Our goals are to examine the ability of HSC-derived osteo-chondrogenic progenitors (compared to MSCs) to serve as a therapeutic modality during complicated non-union fractures and fractures during SSRI administration, uncover the mechanisms by which these cells may have a beneficial effect, and elucidate the effects of SSRIs on this unique population of progenitor cells. Specific Aims are to determine 1) if modulation of HSC-derived osteo-chondrogenic precursors in vivo promotes healing in a model of atrophic non-union and 2) if SSRIs impair fracture healing via inhibition of HSC-derived osteogenic precursors. These studies are significant in that they challenge existing dogma by suggesting a novel HSC origin for bone cells and innovative in that they will determine the potential of exploiting HSCs to enhance healing in cases of complicated fracture. Findings from this study are easily translated to clinic (HSC mobilization is FDA approved for other applications) and have the potential to directly impact Veteran Health Care. Understanding how SSRIs affect bone healing and specific stem cell compartments may have tremendous clinical impact given the large number of injured Veterans prescribed SSRIs. This work will have far-reaching benefit for military personnel and Veterans at high risk for complicated fracture, specifically those of OIF/OEF/OND being treated for clinical depression and/or PTSD using an SSRI, and thus, is highly relevant to the VA Mission.