Adhesion structures and their cytoskeleton-membrane interactions at podosomes as a specific signal transduction site of osteoclasts

利用细胞剪切、旋转复制和荧光细胞化学技术研究了破骨细胞足小体中细胞骨架的组织结构。剪切后，在膜暴露的细胞质侧，留下了与细胞骨架相关的网格蛋白斑块和颗粒。足小体的细胞骨架以两种类型的肌动蛋白丝为特征，即在足小体核心周围的部分有较长的丝，在核心处有高度分枝的短丝。从足小体放射出的单个肌动蛋白丝与沿着丝的长度的几个膜颗粒相互作用。颗粒沿着单个肌动蛋白丝的长度与膜表面进行了许多侧向接触。足小体中肌动蛋白丝的极性变得定向，其倒钩末端指向足小体的核心。肌动蛋白细胞骨架在足小体处终止或分支，在足小体处膜紧密地粘附在基质上。微管不是由通常存在于足体结构中的微管组成；然而，某些微管在形态上似乎与足囊体核心直接接触。大多数较大的网格蛋白斑块由平面的网格蛋白晶格组成，这些网格蛋白晶格相互连接，形成广泛的网格蛋白区域。然而，小的深内陷的网格蛋白斑块和足囊体细胞骨架位于一起。因此，这些观察结果表明，破骨细胞腹侧膜上的网格蛋白斑块可能参与细胞粘附和受体-配体复合物的形成。

Organization of the cytoskeleton in podosomes of osteoclasts was studied by use of cell shearing, rotary replication, and fluorescence cytochemical techniques. After shearing, on the exposed cytoplasmic side of the membrane, clathrin plaques and particles associated with the cytoskeleton were left behind. The cytoskeleton of the podosomes was characterized by two types of actin filaments, which were identified, i.e., relatively long filaments in the portion surrounding the podosome core, and highly branched short filaments in the core. Individual actin filaments radiating from the podosomes interacted with several membrane particles along the length of the filaments. Many lateral contacts with the membrane surface by the particles were made along the length of individual actin filaments. The polarity of actin filaments in podosomes became oriented such that their barbed ends were directed towards the core of podosomes. The actin cytoskeletons terminated or branched at the podosomes, where the membrane tightly adhered to the substratum. Microtubules were not composed of usually present in the podosome structures ; however, certain microtubules appeared to be morphologically in direct contact with the podosome core. Most of the larger clathrin plaques consisted of flat sheets of clathrin lattices that interconnected neighboring clathrin lattices to form an extensive clathrin area. However, the small deeply invaginated clathrin plaques and the podosomal cytoskeleton were located close together. Consequently, these observations indicate that the clathrin plaques on the ventral membrane of osteoclasts might be involved in both cell adhesion and formation of receptor-ligand complexes.