Molecular functions of Shank2 and Shank3 in bone integrity and osteoporosis

骨质疏松症是老龄化社会的一种退行性疾病，是卫生系统的重大负担。为了提高骨质量，需要新的有利于成骨细胞及其衍生骨细胞的药物靶点。我们发现了成骨细胞分化和功能的新调节因子，也可能在原发性成骨细胞的siRNA筛选中导致骨质疏松症治疗的新靶点。我们发现了Shank家族蛋白（特别是Shank3）及其许多成骨细胞分化所需的相互作用因子。柄蛋白定位于兴奋性突触的突触后密度，是主要的支架分子。已知Shank突变是导致自闭症谱系神经精神障碍的原因，但在一些情况下也会导致身材矮小和面部畸形，这表明人类骨骼稳态的Shank相关失调。因此，我们发现敲除Shank2/3基因的小鼠有明显的骨质减少。我们进一步发现，在未分化的成骨细胞中，Shank3是核定位的，随后在成骨细胞分化过程中，在细胞的细胞质和皮层下区域发现。我们假设从突触中发现的支架蛋白在骨形成中起着重要作用，随着年龄的增长，它们的功能会下降。我们进一步假设，这些蛋白质的不同亚细胞定位调节分化过程。我们通过分析化合物敲除小鼠、它们的原代细胞和源自Phelan McDermid综合征患者携带Shank3基因突变的iPS细胞的人成骨细胞来解决这个问题。最后，我们将验证用于治疗Phelan McDermid患者的治疗方案可能会改善骨形成的假设。预期的结果将深入了解费伦麦克德米综合征患者骨骼异常的分子机制，以及骨质疏松症等骨骼疾病。

Osteoporosis is a degenerative disease of the aging society, which is a significant burden for the health system. To increase bone quality novel drug targets favoring bone forming osteoblasts and their derived osteocytes are required. We identified novel regulators of osteoblast differentiation and function that could also lead to novel targets in osteoporosis treatment with a siRNA screen in primary osteoblasts. We found Shank family proteins (in particular Shank3) and many of their interacting factors required for osteoblast differentiation. Shank proteins localize to postsynaptic densities of excitatory synapses and are major scaffolding molecules. Shank mutations are known to be causative for neuropsychiatric disorders of the autism spectrum but in several cases also lead to short stature and facial dysmorphias indicating a Shank related misregulation of bone homeostasis in humans. Accordingly, we found that knockout mice for Shank2/3 have a dramatic osteopenia. We further detected that in undifferentiated osteoblasts Shank3 is nuclear localized and is found subsequently in cytoplasm and subcortical regions of the cell during osteoblast differentiation. We hypothesize that scaffolding proteins known from synapses have an essential role for bone formation and that during ageing their function decreases. We further hypothesize that distinct subcellular localizations of these proteins are regulating the differentiation process. We address this by the analysis of compound knockout mice, their primary cells and human osteoblasts derived from iPS cells of Phelan McDermid syndrome patients carrying mutations within Shank3 gene. Finally, we will test the hypothesis that treatment regimens used to treat Phelan McDermid patients might lead to improved bone formation. The expected results will give insights into molecular mechanisms of the bone abnormalities of Phelan McDermid syndrome patients as well as for bone disorders, such as osteoporosis in general.