Osteoclasts’ Ability to Generate Trenches Rather Than Pits Depends on High Levels of Active Cathepsin K and Efficient Clearance of Resorption Products

Abstract

Until recently, it was well-accepted that osteoclasts resorb bone according to the resorption cycle model. This model is based on the assumption that osteoclasts are immobile during bone erosion, allowing the actin ring to be firmly attached and thereby provide an effective seal encircling the resorptive compartment. However, through time-lapse, it was recently documented that osteoclasts making elongated resorption cavities and trenches move across the bone surface while efficiently resorbing bone. However, it was also shown that osteoclasts making rounded cavities and pits indeed resorb bone while they are immobile. Only little is known about what distinguishes these two different resorption modes. This is of both basic and clinical interest because these resorption modes are differently sensitive to drugs and are affected by the gender as well as age of the donor. In the present manuscript we show that: 1. levels of active cathepsin K determine the switch from pit to trench mode; 2. pit and trench mode depend on clathrin-mediated endocytosis; and 3. a mechanism integrating release of resorption products and membrane/integrin recycling is required for prolongation of trench mode. Our study therefore contributes to an improved understanding of the molecular and cellular determinants for the two osteoclastic bone resorption modes.

Increased Bone Resorption during Lactation in Pycnodysostosis

“…Though not measured, it seems feasible that due the huge size differences between the trenches formed by the control and the tiny pits formed in the case of pycnodysostosis, the latter were much shallower. In a recent study the same group investigated differences between pit-and trench-resorption modes by osteoclasts formed in cultures of human CD14 + monocytes, similar to the method used in our study [13]. They found a highly significant correlation between the levels of active cathepsin K and the prevalence of trench formation in a cohort of 14 donors and confirmed the importance of cathepsin K in the switch from pit to trench mode.…”

“…Their conclusions about the effects of cathepsin K in the formation of different types of resorption cavities-based on experiments in which cathepsin K levels in normal osteoclasts were modified by treatment with the cathepsin K inhibitor odanacatib-are in agreement with the direct demonstration of the inability of cathepsin K-deficient human osteoclasts to form trenches on bovine bone slices reported here. However, CTX concentrations have not been measured in the culture media, not in the study of Borggaard et al [13] nor in our study. Such a measurement was, however, reported in a study of a 55-year-old woman with pycnodysostosis and psoriatic arthritis [29].…”

“…As shown in Figure 1 C, osteoclasts from the healthy donor formed resorption trenches on cortical bone slices, whereas osteoclasts from the patient showed resorption pits but no trenches ( Figure 1 D,H). Trench formation has been associated with high collagenolytic activity, higher cathepsin K expression, and deeper demineralization [ 13 , 14 ].…”

The Mechanism Switching the Osteoclast From Short to Long Duration Bone Resorption

“…Common experience is that osteoclasts seeded on bone slices (obtained by sawing and therefore exposing mineralized bone Søe et al, 2013;Merrild et al, 2015;Panwar et al, 2016Panwar et al, , 2017Borggaard et al, 2020 • Decreased by bisphosphonates Møller et al, 2020a • Appears to favor coupling with bone formation Delaisse et al, 2020 See section 1 for the definition of pit and trench modes. *Note that in a typical in vitro experiment, (performed with osteoclasts generated from CD14-positive monocytes from blood donors) 90% of the trench formation events start as a pit which is thereafter enlarged laterally, whereas half of the resorption events do not go beyond the formation of a pit (Søe and Delaissé, 2017).…”

“…The need for sufficient cathepsin K to allow transition from pit to trench mode is clearly demonstrated by the concomitant abolishment of trench formation and increase in pit formation in response to cathepsin K inhibitors (Søe et al, 2013;Panwar et al, 2016Panwar et al, , 2017Borggaard et al, 2020), as mentioned in section 3.2. It is interesting to note the relevance of this finding to the variations The small arrows originating from the SZ indicate the surface that must be devoid of collagen to allow the displacement of the SZ.…”