Conditional ablation of Meningioma-1 in osteoblasts

描述（由申请人提供）：维生素D受体（VDR）及其必需的激素配体1,25-二羟基维生素D3 (1,25(OH)2D3)，以及一系列核受体共激活因子和共抑制因子协调控制选择基因和基因网络的表达，通过增加肠道钙吸收和直接影响成骨细胞的增殖、分化和功能来影响骨骼。脑膜瘤-1 （MN1）是成骨细胞中由VDR和1,25(OH)2D3调控的靶基因之一。MN1是VDR和其他核受体的辅激活剂，是VDR充分发挥配体诱导活性所必需的。MN1在骨骼生物学中的重要意义是由MN1缺失小鼠模型的表型提示的，该模型在膜内骨化中显示颅骨骨骼异常。体外数据支持这种新因子在成骨细胞和骨骼生物学中的重要作用。例如，MN1在包括原代成骨细胞在内的许多成骨细胞系中表达，其表达在成骨细胞分化过程中增加，在共培养研究中，MN1消融显著影响成骨细胞形态、成骨细胞增殖、成骨细胞分化、1,25(OH) 2d3激活基因表达和1,25(OH) 2d3刺激的破骨细胞形成。然而，这些体外方法不能解决MN1在出生后骨骼中的生理相关性。由于全球MN1KO模型的出生后致死率，骨架中MN1生物学的这一关键点仍未得到解决。在当前的提案中，我们提出了一种解决这一问题的新小鼠模型的开发的初步数据。我们的实验室利用Col¿1(I)-cre缺失菌株与小鼠MN1基因外显子1两侧有loxP位点的条件等位基因结合，开发了一种成骨细胞靶向消融小鼠MN1基因的小鼠。Col¿1(I)-cre+MN1fl/fl小鼠在20周龄时表现出性别选择性和区室选择性的低矿化骨骼表型。这些缺陷让人想起在人类和啮齿动物模型中正常衰老过程中观察到的年轻人早期骨质流失。这些新数据暗示MN1在更广泛的骨骼重塑中独立于VDR和维生素D内分泌系统。这个最新的模型有可能揭示年轻成年哺乳动物早期骨小梁丢失的潜在机制和生物学的新见解。因此，本提案的整体单一研究目标是表征这种新模型的详细骨骼表型，并确定MN1在出生后骨骼生理学中的意义。

DESCRIPTION (provided by applicant): The vitamin D receptor (VDR), its requisite hormonal ligand, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), as well as a host of nuclear receptor coactivators and corepressors coordinately control the expression of select genes and gene networks to impact bone by increasing intestinal calcium absorption and by directly affecting osteoblast proliferation, differentiation, and function. Meningioma-1 (MN1) is one such target gene that VDR and 1,25(OH)2D3 regulate in osteoblasts. MN1 is a coactivator of VDR and other nuclear receptors and it is needed for full ligand-induced activity of the VDR. The significance of MN1 in skeletal biology is suggested by the phenotype of the MN1 null mouse model which displays cranial skeletal anomalies in intramembranous ossification. In vitro data support an important role for this novel factor in the osteoblast and in skeletal biology. For example, MN1 is expressed in numerous osteoblastic cell lines including primary osteoblasts, its expression is increased during osteoblast differentiation, MN1 ablation dramatically impacts osteoblast morphology, osteoblast proliferation, osteoblast differentiation, 1,25(OH)2D3-activated gene expression, and 1,25(OH)2D3-stimulated osteoclastogenesis in co-culture studies. However, these in vitro approaches do not address the physiological relevance of MN1 in the postnatal skeleton. Due to postnatal lethality of the global MN1KO model, this critical poin of MN1 biology in the skeleton has remained unaddressed. In this current proposal, we present preliminary data on the development of a new mouse model that addresses this issue. Our laboratory developed a mouse with osteoblast-targeted ablation of the murine MN1 gene using the Col¿1(I)-cre deletor strain combined with a conditional allele in which exon 1 of the murine MN1 gene is flanked by loxP sites. The Col¿1(I)-cre+MN1fl/fl mice displayed a gender-selective and compartment-selective undermineralized skeletal phenotype at 20 weeks of age. These defects are reminiscent of those observed in early bone loss in young adults during the normal aging process in human and rodent models. These new data implicate MN1 in more global skeletal remodeling independent of VDR and the vitamin D endocrine system. This newest model has the potential to uncover new insight into potential mechanisms and biologies involved in early trabecular bone loss in young, adult mammals. Thus, the overall single research goal of this proposal is to characterize the detailed skeletal phenotype of this new model and establish the significance of MN1 in postnatal skeletal physiology.