RANKL and lymphocyte-mediated bone loss

描述（由申请人提供）： 雌激素水平的下降，如发生在绝经期，通过增加骨再吸收破骨细胞的数量引起骨丢失。雌激素控制破骨细胞数量的机制仅部分了解，但以前的研究表明淋巴细胞起着重要作用。例如，小鼠或大鼠的卵巢切除术始终导致骨髓中B淋巴细胞数量增加。已表明这种B细胞数量的增加通过不同机制促进破骨细胞形成的增加，例如B细胞产生破骨细胞生成细胞因子NF-κ-B配体受体激活剂（RANKL）和B细胞前体分化为破骨细胞。然而，直到最近，没有功能性证据表明B细胞在卵巢切除术诱导的骨丢失中起重要作用。在导致该应用的研究中，我们发现，由B淋巴细胞产生的NF-κ-B配体（RANKL）的细胞因子受体激活剂对于雌激素缺乏引起的小鼠松质骨丢失是必需的。重要的是，雌激素缺乏引起的B细胞数量增加也需要RANKL。此外，卵巢切除术未增加野生型小鼠B细胞中RANKL的水平。总之，这些结果表明，这是增加B细胞数量所需的卵巢切除术诱导的骨丢失在这个模型中。同样重要的是要注意，从B细胞中删除RANKL并不能防止雌激素缺乏引起的皮质骨丢失。因此，由B细胞以外的细胞类型产生的RANKL必须参与该骨骼隔室中的破骨细胞形成。基于这些结果，我们假设雌激素的丢失导致松质骨丢失，部分是通过增加B细胞的数量，然后B细胞可以作为破骨细胞祖细胞。此外，我们认为雌激素的丢失通过改变成骨细胞系细胞RANKL的产生而导致皮质骨丢失。为了解决这些假设，将进行谱系追踪研究，以确定B细胞在其发育的任何阶段是否可以在体内分化成骨吸收破骨细胞。此外，雌激素是否通过直接作用于这些细胞来抑制B细胞数量将通过从该细胞类型中有条件地删除雌激素受体α来确定。最后，将从成骨细胞谱系的骨细胞或基质细胞中删除RANKL基因的小鼠进行卵巢切除，以确定这些细胞类型产生的RANKL是否有助于雌激素缺乏引起的皮质骨丢失。

DESCRIPTION (provided by applicant): A decline in estrogen levels, such as occurs at the menopause, causes bone loss by increasing the number of bone resorbing osteoclasts. The mechanisms by which estrogen controls osteoclast number are only partially understood, but previous studies suggest that lymphocytes play an important role. For example, ovariectomy of mice or rats consistently leads to increased numbers of B lymphocytes in the bone marrow. This increase in B cell number has been suggested to contribute to increased osteoclast formation by different mechanisms, such as B cell production of the osteoclastogenic cytokine receptor activator of NF-kappa-B ligand (RANKL) and differentiation of B cell precursors into osteoclasts. However, until recently, there was no functional evidence that B cells play an essential role in ovariectomy-induced bone loss. In studies leading to this application, we have found that production of the cytokine receptor activator of NF-kappa-B ligand (RANKL) by B lymphocytes is essential for the cancellous bone loss caused by estrogen deficiency in mice. Importantly, RANKL is also required for the increase in B cell number that is caused by estrogen deficiency. Also, ovariectomy did not increase the levels of RANKL in B cells in wild type mice. Together, these results suggest that it is the increase in B cell number that is required for ovariectomy-induced bone loss in this model. It is also important to note that deletion of RANKL from B cells did not prevent loss of cortical bone caused by estrogen deficiency. Therefore, RANKL produced by cell types other than B cells must be involved in the osteoclast formation in this skeletal compartment. Based on these results, we hypothesize that loss of estrogen causes cancellous bone loss, in part, by increasing the number of B cells, which can then act as osteoclast progenitors. Further, we propose that loss of estrogen causes cortical bone loss by altering production of RANKL by cells of the osteoblast lineage. To address these hypotheses, lineage-tracing studies will be performed to determine whether B cells, at any stage of their development, can differentiate into bone resorbing osteoclasts in vivo. In addition, whether estrogen suppresses B cell number by acting directly on these cells will be determined by conditional deletion of estrogen receptor alpha from this cell type. Lastly, mice in which the RANKL gene has been deleted from either osteocytes or from stromal cells of the osteoblast lineage will be ovariectomized to determine whether RANKL produced by these cell types contributes to the cortical bone loss caused by estrogen deficiency.