The Sealing Zone in Osteoclasts: A Self-Organized Structure on the Bone

Abstract

Osteoclasts form a specialized cell-matrix adhesion structure, known as the "sealing zone", during bone resorption. The sealing zone is a dynamic actin-rich structure that defines the resorption area of the bone. The detailed dynamics and fine structure of the sealing zone have been elusive. Osteoclasts plated on glass do not form a sealing zone, but generate a separate supra-molecular structure called the "podosome belt". Podosomes are integrin-based adhesion complexes involved in matrix adhesion, cell migration, matrix degradation, and mechanosensing. Invadopodia, podosome-like protrusions in cancer cells, are involved in cell invasion into other tissues by promoting matrix degradation. Both podosomes and invadopodia exhibit actin pattern transitions during maturation. We previously found that Arp2/3-dependent actin flow occurs in all observed assembly patterns of podosomes in osteoclasts on glass. It is known that the actin wave in Dictyostelium cells exhibits a similar pattern transition in its evolution. Because of significant advances in our understanding regarding the mechanism of podosomes/invadopodia formation over the last decade, we revisited the structure and function of the sealing zone in this review, highlighting the possible involvement of self-organized actin waves in the organogenesis of the sealing zone.

Keywords: Arp2/3; actin polymerization; actin wave; integrin; invadopodia; osteoclasts; plasma membrane; podosome; sealing zone.

Figure 1

Polarization of osteoclast during bone resorption. The scheme on the left shows a cross section of the resorbing osteoclast in the stationary mode. The polarization of an osteoclast compartmentalizes its plasma membrane on the bone [9]. The resorption lacune is the space enclosed within the sealing zone, the ruffled border membrane, and the bone. The stationary resorbing osteoclast transits into one of the two migratory modes. The osteoclast stops bone resorption and migrates on bone (upper panel in the middle), or continues bone resorption in the migratory mode (lower panel in the middle). Green arrows indicate the direction of movement. The former mode of osteoclasts forms pit-type resorption traces (upper panel in the right), while the latter produces trail- or trench-type resorption traces (lower panel in the right) [14,15]. The right panels show the resorption traces in a bird’s eye view. BL, basolateral membrane; FSD, functional secretory domain; N, nucleus; RB, ruffled border membrane; RL, resorption lacune; SZ, sealing zone.

Figure 2

The height of F-actin-rich domains of the podosome-related structures of osteoclasts. Osteoclasts differentiated from RAW 264.7 cells on glass were stained with FITC (fluorescein isothiocyanate)-phalloidin for N-structured illumination microscopy (SIM) imaging. Z-stacks of the entire FITC-positive volume were acquired at 0.3 µm intervals. The images are representatives of cross sections of the podosome cluster, the podosome ring, the podosome belt, and the zipper-like structure.

Figure 3

Model of the podosome field. The model illustrates a perspective of the basic structural elements of the podosome field in the pattern of the podosome cluster. The podosome field represents a distinct entity in the plasma membrane and comprises the actin cores, a network of F-actin cables, integrin islets, and the actin flow. The actin flow is generated by Arp2/3-dependent branched actin elongation. Pattern transitions of the podosome field may produce the podosome cluster, the podosome ring, the podosome belt, and the zipper-like structure in osteoclasts.

Figure 4

Pattern transition of actin assembly. A bird’s-eye view of the various actin assemblies. The assembly pattern of the actin core evolves from a cluster, to a ring, to a belt in invadopodia in RSV-BHK cells [28] and in osteoclasts on glass [21]. In osteoclasts on apatite–collagen-coated glass [24] and on dentin [38], a ring-like sealing zone evolves from the uncharacterized actin patch. Most crescent-like sealing zones (80%) on bone evolve from a ring-shaped sealing zone, whereas a small number of them (20%) directly develops from the actin patch [15]. In Dictyostelium cells, the actin wave takes various patterns including a circular ring, an arc shape, and a belt [39]. The actin wave originates from the actin clusters embedded in a PtdIns(3,4,5)P3 patch. The wave does not contain the actin core.