Acidification of the Osteoclastic Resorption Compartment Provides Insight into the Coupling of Bone Formation to Bone Resorption

Abstract

Patients with defective osteoclastic acidification have increased numbers of osteoclasts, with decreased resorption, but bone formation that remains unchanged. We demonstrate that osteoclast survival is increased when acidification is impaired, and that impairment of acidification results in inhibition of bone resorption without inhibition of bone formation. We investigated the role of acidification in human osteoclastic resorption and life span in vitro using inhibitors of chloride channels (NS5818/ NS3696), the proton pump (bafilomycin) and cathepsin K. We found that bafilomycin and NS5818 dose dependently inhibited acidification of the osteoclastic resorption compartment and bone resorption. Inhibition of bone resorption by inhibition of acidification, but not cathepsin K inhibition, augmented osteoclast survival, which resulted in a 150 to 300% increase in osteoclasts compared to controls. We investigated the effect of inhibition of osteoclastic acidification in vivo by using the rat ovariectomy model with twice daily oral dosing of NS3696 at 50 mg/kg for 6 weeks. We observed a 60% decrease in resorption (DPYR), increased tartrate-resistant acid phosphatase levels, and no effect on bone formation evaluated by osteocalcin. We speculate that attenuated acidification inhibits dissolution of the inorganic phase of bone and results in an increased number of nonresorbing osteoclasts that are responsible for the coupling to normal bone formation. Thus, we suggest that acidification is essential for normal bone remodeling and that attenuated acidification leads to uncoupling with decreased bone resorption and unaffected bone formation. The coupling process is understood as a bone formation response that is the consequence of bone resorption, with an amount of bone formed that is equal to that resorbed. 1,2 Uncoupling occurs when the balance between formation and resorption is disturbed, which might lead to either osteopetrosis or osteoporosis. 3,4 Although it has long been appreciated that bone formation is tightly coupled to bone resorption in normal adult bone turnover, 5,6 this coupling can be dissociated in some circumstances, for example during skeletal growth and in some but not all osteopetrotic mutations.Bone consists of two phases, the organic phase, which contains proteins such as collagen type I, and the inorganic phase, which consists of crystallized calcium phosphate. Dissolution of the inorganic phase of bone under the osteoclast attached to the bone surface is a prerequisite for degradation of the organic phase, and is mediated by acidification of the resorption compartment. The decrease in pH necessary to dissolve the inorganic phase is mediated by an active transport of protons over the ruffled border membrane, which is driven by an osteoclastic Vtype H ϩ ATPase. 7,8 At the same time a passive transport of chloride through chloride channels preserves the electroneutrality, 9 and is mediated by the chloride channel ClC-7. 10,11 Degradation of the organic phase of bone is for the major part mediated b...

Genetics in Endocrinology: Autosomal dominant osteopetrosis revisited: lessons from recent studies

“…The osteoclasts have an attenuated capacity to resorb the calcified bone matrix. This mechanism, at least partially, seems to have an auto-regulatory effect of calcium directly on osteoclast survival (63,66). However, other studies have also shown that release of transforming growth factor b from the bone matrix caused induction of osteoclast apoptosis (67) and thus also participates in this auto-regulatory control of osteoclast activity.…”

“…Histomorphometric analyses of ADO and osteoclast-rich ARO patients showed increased numbers of very large osteoclasts in vivo (17,41). In vitro analyses showed that osteoclastogenesis was normal, both with respect to time frame and numbers of osteoclasts, and with respect to morphology (17,52,53,63,64). The underlying reason for the lack of bone resorption by these osteoclasts was shown to be reduced acid secretion into the resorption lacunae (52,53,63,64,65), an effect also seen in ClC-7-deficient mice (49).…”

“…In addition to increased survival due to decreased release of pro-apoptotic factors from the mineral, increases in parathyroid hormone could contribute to the increased number of osteoclasts (68). However, this increase in survival is also seen in vitro in pure cultures of osteoclasts where acid secretion is reduced due to mutations or blocked pharmacologically hence showing at least some PTH-independent osteoclast survival effects (63,66,69).…”

Differentially expressed genes in autosomal dominant osteopetrosis type II osteoclasts reveal known and novel pathways for osteoclast biology

“…It has been proposed that CLCN7 inhibition could be used to treat osteoporosis. 64 However, increased bone formation has never been demonstrated so far in ADO II. Histomorphometry has evidenced normal bone-forming surfaces in very few patients with ADO II.…”

“…This fits in with the hypothesis that osteoclasts present on bone surfaces can synthesize molecules that directly stimulate bone formation. 64,65 Recently sphingosine-1-phosphate (S1P), among other factors secreted by osteoclasts, has been shown to stimulate osteoblast differentiation. 65 More recently, in mice deficient for CTSK, SPHK1 increased expression triggers an higher secretion of S1P by osteoclasts, which has been shown to be responsible for increased bone formation.…”