Nanoscale architecture and coordination of actin cores within the sealing zone of human osteoclasts

Abstract

Osteoclasts are unique in their capacity to degrade bone tissue. To achieve this process, osteoclasts form a specific structure called the sealing zone, which creates a close contact with bone and confines the release of protons and hydrolases for bone degradation. The sealing zone is composed of actin structures called podosomes nested in a dense actin network. The organization of these actin structures inside the sealing zone at the nano scale is still unknown. Here, we combine cutting-edge microscopy methods to reveal the nanoscale architecture and dynamics of the sealing zone formed by human osteoclasts on bone surface. Random illumination microscopy allowed the identification and live imaging of densely packed actin cores within the sealing zone. A cross-correlation analysis of the fluctuations of actin content at these cores indicates that they are locally synchronized. Further examination shows that the sealing zone is composed of groups of synchronized cores linked by a-actinin1 positive filaments, and encircled by adhesion complexes. Thus, we propose that the confinement of bone degradation mediators is achieved through the coordination of islets of actin cores and not by the global coordination of all podosomal subunits forming the sealing zone.

Moesin activation controls bone resorption and tunneling nanotube-dependent osteoclast fusion

“…In addition to regulating osteoclast fusion, moesin also influences the number and architecture of the sealing zones. These structures are composed of a dense network of podosomes organized in clusters 19,21 and as defined in macrophages, each individual podosome can exert a protrusion force on the substrate that is correlated to the F-actin content 102 . Thus, the increased width of the podosome-rich zone observed upon moesin depletion may favor more efficient sealing of osteoclasts to the bone and therefore increase the concentration of bone-degradative molecules in the resorption area 19,86 .…”

“…Upon attachment to bone, mature osteoclasts form an F-actin rich structure crucial for bone resorptive activity called the sealing zone. This bone-anchored adhesion structure demarcates the area of bone resorption from the rest of the environment and consists of a complex assembly of podosomes [19][20][21][22][23] . Each of these steps of osteoclastogenesis involves rearrangements of the actin cytoskeleton, but, the precise mechanism(s) and sequence of events still remain poorly understood 16,24 .…”

Harnessing osteoimmunity to treat peri-implant inflammatory osteolysis

“…Podosomes, which function to degrade the extracellular matrix (ECM), are found in cells such as monocytes, macrophages, osteoclasts, and migratory dendritic cells. [122][123][124][125] It is involved in physiological processes, including cell migration, bone remodeling, and angiogenesis. [126][127][128] Unlike podosomes in macrophages or dendritic cells, which are dispersed on the ventral adhesion side or at the tip of the migration direction, podosomes gradually evolve from clusters and rings to belts (on glass) or sealing zones (on bone) during osteoclast development.…”

Bone resorption by osteoclasts involves fine tuning of RHOA activity by its microtubule-associated exchange factor GEF-H1

“…For optimal bone resorption efficiency, osteoclasts form a specialized cell-matrix adhesion structure, called sealing zone or actin ring that creates a confined acidic microenvironment over the area that needs to be resorbed. This dynamic actin-rich structure is composed of densely packed podosomes locally connected by acto-myosin filaments and surrounded by adhesion complexes (Luxenburg et al, 2007;Portes et al, 2022). It is intimately connected to a network of microtubules (Akisaka et al, 2011), and microtubule depolymerization in osteoclasts disrupts the sealing zone and inhibits their resorption activity (Destaing et al, 2003;Okumura et al, 2006).…”