Impact of Mild TBI on Bone Formation

 描述(由申请人提供)： 创伤性脑损伤涉及外力对大脑的损伤，每年在退伍军人和普通人群中都会造成大量死亡和永久性残疾。脑外伤是美国最常见的神经学诊断，已被确定为严重的公共卫生问题。虽然人们正在广泛研究头部原发损伤对最初几个小时和几天内发生的病理生理和神经化学事件的影响和机制，但对于脑损伤对远端器官的长期影响知之甚少，这些器官由下丘脑通过脑下垂体控制。在这份功绩评估建议中，我们选择了基于以下基本原理来研究脑外伤对骨骼系统的影响。最近，神经信号对骨的调节的发现代表了一个新兴的研究领域，即识别神经系统和骨细胞之间的新的调节轴。在这方面，众所周知，大脑的下丘脑区通过脑下垂体控制内分泌系统。值得注意的是，下丘脑分泌生长激素释放激素(GHRH)，作用于脑下垂体调节生长激素(GH)的产生，生长激素是调节骨骼新陈代谢的重要激素。据估计，30-50%的颅脑损伤患者患有垂体功能减退。此外，最近的研究表明，瘦素和神经肽在骨量的中枢控制中发挥着重要作用。基于下丘脑核团细胞在神经(内分泌)骨调节中的重要作用 重塑，可以预见，对大脑的损伤将对控制骨骼生长和维护的调节分子产生严重影响。基于上述理论基础，我们建议在本研究中检验以下假设：1)即使是轻微的颅脑损伤，也会对骨骼的生长和维持以及骨骼修复受损组织的能力产生长期的负面影响。2)颅脑损伤对骨形成(BF)的抑制作用部分是通过下调GH/胰岛素样生长因子-I轴实现的。3)颅脑损伤夸大了骨骼卸载对高炉的负面影响。为了验证这些假设，我们将使用最近建立的人类重复性轻度颅脑损伤的小鼠模型，该模型利用减重方法制造闭合性头部损伤。我们建议使用轻度脑损伤模型，因为在美国每年报告的170万例脑损伤病例中，大约85%代表轻度病例。在我们的初步研究中，我们验证了轻度脑外伤小鼠模型在骨骼研究中的有效性。为了确定轻度颅脑损伤是否会对骨骼再生损伤的能力产生负面影响，我们将建立一个标准的闭合性股骨骨折模型，并使用显微CT、组织学和基因表达终点来评估修复情况。为了评估GH/IGF-I轴的作用，我们将通过免疫组织化学方法确定脑创伤小鼠GH/IGF-I水平的变化是否是由于下丘脑GHRH表达减少所致。为了确定GH/IGF生成减少是否导致小鼠BF受损，我们将给TBI小鼠注射GH，并评估GH替代治疗是否挽救了TBI引起的BF变化和骨再生。我们将利用生长激素缺乏的LIT/LIT小鼠模型来评估生长激素在颅脑损伤诱导的骨丢失中的作用。我们还将确定颅脑损伤和骨骼卸载是否会导致比任何一种情况下更大的BF缺陷，因为颅脑损伤患者在受伤后立即长时间卧床，而且骨骼卸载通过抑制IGF-I信号通路的激活而诱导对IGF-I的抵抗。我们将用尾部悬吊后肢抬高模型进行骨骼卸载，并通过显微CT、组织形态计量学或基因表达分析评估骨骼参数。我们相信，我们对所提出的假说的成功确认将导致新的治疗策略的开发，以促进脑外伤患者的长期健康。

 DESCRIPTION (provided by applicant): Traumatic brain injury (TBI), which involves damage to the brain from an external force, contributes to a substantial number of deaths and cases of permanent disability both in the Veteran population as well as in the general population each year. TBI is the most common neurological diagnosis in the U.S. and has been identified as a serious public health problem. While the consequences and mechanisms of primary injury to the head on the pathophysiological and neurochemical events that occur during the course of initial hours and days are being extensively investigated, little is known on the long term consequence of TBI on remote organs that are under hypothalamic control via the pituitary. In this merit review proposal, we have chosen to study the TBI effect on the skeletal system based on the following rationale. Recently, the discovery of bone regulation by neural signals represents an emerging area of study that is identifying novel regulatory axes between the nervous system and bone cells. In this regard, it is well known that the hypothalamic region of the brain controls the endocrine system via the pituitary. Notably, the hypothalamus secretes growth hormone releasing hormone (GHRH) which acts on the pituitary gland to regulate production of growth hormone (GH), an important hormone that regulates skeletal metabolism. It is estimated that 30-50% of TBI patients suffer from hypopituitarism. Besides, recent studies demonstrate an important role for central control of bone mass involving leptins and neuropeptides. Based on the important role for cells of the hypothalamic nuclei in the neuro (endo)crine regulation of bone remodeling, it is predictable that injury to the brain will have a severe impact on the regulatory molecules that control skeletal growth and maintenance. Based on the above rationale, we propose to test the following hypotheses in this study: 1) TBI, even in its milder form, exerts lon lasting negative effects on bone growth and maintenance and the ability of skeleton to repair the damaged tissue in response to injury. 2) The attenuating effects of TBI on bone formation (BF) are mediated in part via down regulation of the GH/insulin-like growth factor-I axis. 3) TBI exaggerates the negative effects of skeletal unloading on BF. To test these hypotheses, we will use a recently established mouse model of human repetitive mild TBI which utilizes the weight drop method to create a closed head injury. We have proposed to use the mild TBI model as approximately 85% of the 1.7 million cases of TBI reported annually in the United State represent mild cases. In our preliminary studies, we have validated the usefulness of the mild TBI mouse model for studies on bone. In order to determine if mild TBI exerts negative effects on the ability of the skeleton to regenerate injuries, we will induce a standard closed femoral fracture and evaluate repair using micro-CT, histology and gene expression end points. To evaluate the role of the GH/IGF-I axis, we will determine if changes in GH/IGF-I levels in the TBI mice are caused by reduced expression of GHRH in the hypothalamus by immunohistochemistry. To determine if reduced GH/IGF production contributes to impaired BF in mice, we will administer GH to TBI mice and evaluate if TBI-induced changes in BF and bone regeneration are rescued by GH replacement. We will utilize a GH deficient lit/lit mouse model to evaluate the role of GH in TBI-induced bone loss. We will also determine if TBI and skeletal unloading lead to greater BF deficits than either condition alone since TBI patients are bedridden for extended periods immediately post injury and since skeletal unloading induces resistance to IGF-I by inhibiting activation of the IGF-I signaling pathways. We will subject TBI and control mice to skeletal unloading using the tail suspension hind limb elevation model or to normal loading and evaluate skeletal parameters by micro-CT, histomorphometry or gene expression analyses. We believe that our successful confirmation of proposed hypotheses will lead to development of new therapeutic strategies to promote long-term health of TBI patients.