The transient appearance of zipper-like actin superstructures during the fusion of osteoclasts

Abstract

SummaryMultinucleated osteoclasts are responsible for bone resorption. Hypermultinucleated osteoclasts are often observed in some bone-related diseases such as Paget's disease and cherubism. The cellular mechanics controlling the size of osteoclasts is poorly understood. We introduced EGFP-actin into RAW 264.7 cells to monitor actin dynamics during osteoclast differentiation. Before their terminal differentiation into osteoclasts, syncytia displayed two main types of actin assembly, podosome clusters and clusters of zipper-like structures. The zipper-like structures morphologically resembled the adhesion zippers found at the initial stage of cell-cell adhesion in keratinocytes. In the zipper-like structure, Arp3 and cortactin overlapped with the distribution of dense F-actin, whereas integrin b3, paxillin and vinculin were localized to the periphery of the structure. The structure was negative for WGA-lectin staining and biotin labeling. The zipper-like structure broke down and transformed into a large actin ring, called a podosome belt. Syncytia containing clusters of zipper-like structures had more nuclei than those with podosome clusters. Differentiated osteoclasts with a podosome belt also formed the zipper-like structure at the cell contact site during cell fusion. The breakdown of the cell contact site resulted in the fusion of the podosome belts following plasma membrane fusion. Additionally, osteoclasts in mouse calvariae formed the zipper-like structure in the sealing zone. Therefore, we propose that the zipper-like actin superstructures might be involved in cell-cell interaction to achieve efficient multinucleation of osteoclasts. Understanding of the zipper-like structure might lead to selective therapeutics for bone diseases caused by hypermultinucleated osteoclasts.

ISG15 Regulates RANKL-Induced Osteoclastogenic Differentiation of RAW264 Cells

“…With respect to its other effects on osteoclastogenesis, ISG15 has recently been shown to disrupt actin architecture that is important for osteoclast fusion. 24,25) Such a line of study supports our finding that ISG15 has a role in osteoclastogenesis. On the other hand, an intracellular free-form of ISG15 and extracellular ISG15 are reported to have important biological functions.…”

Regulation of osteoclasts by membrane-derived lipid mediators

“…Reduced expression of Tks5 results in the loss of circumferential podosomes and cell–cell fusion [60] (Table 1), which is in accordance with recent findings showing that N-WASP-dependent actin-rich protrusive structures are also key drivers of myoblast fusion [99, 100]. Alternatively, circumferential podosome expansions in osteoclasts often accompany tiny membrane protrusions [101] that might be generated by membrane-deforming proteins such as Bin-Amphiphysin-Rvs161/167 (BAR) domain superfamily proteins [102]. For this reason, we refer to the protrusions observed during osteoclast fusion as fusion - competent protrusions .…”

Osteoclast Fusion: Time‐Lapse Reveals Involvement of CD47 and Syncytin‐1 at Different Stages of Nuclearity

“…5C). Although recently a number of studies have focused on the morphologies of fusion partners immediately prior to fusion (Oikawa et al, 2012; Takito and Nakamura, 2012; Takito et al, 2012; Fiorino and Harrison, 2016), knowledge about the molecular composition at these fusion sites is still very scarce. However, considering that syncytin‐1 originates from a retroviral fusion protein it may make sense that it is this fusion route that is affected.…”

“…These analyses are powerful, but are not able to elucidate on the details of individual fusion events leading to this outcome. However, recently analyses of OC fusion in real‐time have been used to identify novel details with regard to the individual fusion partners, their molecular‐ and fusion‐characteristics (Takito and Nakamura, 2012; Takito et al, 2012; Levaot et al, 2015; Soe et al, 2015; Wang et al, 2015; Fiorino and Harrison, 2016). We have recently published a study (Soe et al, 2015), in which we analyzed a large quantity of time‐lapse recordings made over a period of 4 days.…”