Osteopetrorickets due to Snx10 Deficiency in Mice Results from Both Failed Osteoclast Activity and Loss of Gastric Acid-Dependent Calcium Absorption

Abstract

Mutations in sorting nexin 10 (Snx10) have recently been found to account for roughly 4% of all human malignant osteopetrosis, some of them fatal. To study the disease pathogenesis, we investigated the expression of Snx10 and created mouse models in which Snx10 was knocked down globally or knocked out in osteoclasts. Endocytosis is severely defective in Snx10-deficent osteoclasts, as is extracellular acidification, ruffled border formation, and bone resorption. We also discovered that Snx10 is highly expressed in stomach epithelium, with mutations leading to high stomach pH and low calcium solubilization. Global Snx10-deficiency in mice results in a combined phenotype: osteopetrosis (due to osteoclast defect) and rickets (due to high stomach pH and low calcium availability, resulting in impaired bone mineralization). Osteopetrorickets, the paradoxical association of insufficient mineralization in the context of a positive total body calcium balance, is thought to occur due to the inability of the osteoclasts to maintain normal calcium–phosphorus homeostasis. However, osteoclast-specific Snx10 knockout had no effect on calcium balance, and therefore led to severe osteopetrosis without rickets. Moreover, supplementation with calcium gluconate rescued mice from the rachitic phenotype and dramatically extended life span in global Snx10-deficient mice, suggesting that this may be a life-saving component of the clinical approach to Snx10-dependent human osteopetrosis that has previously gone unrecognized. We conclude that tissue-specific effects of Snx10 mutation need to be considered in clinical approaches to this disease entity. Reliance solely on hematopoietic stem cell transplantation can leave hypocalcemia uncorrected with sometimes fatal consequences. These studies established an essential role for Snx10 in bone homeostasis and underscore the importance of gastric acidification in calcium uptake.

Snx10 and PIKfyve are required for lysosome formation in osteoclasts

“…These studies were compliant with all federal and local guidelines. Snx10 floxed mice were described elsewhere . PIKfyve floxed mice (Stock No: 029331) were purchased from The Jackson Laboratory.…”

“…Its overexpression causes dramatic enlargement of endosomes and blocks the endosome‐to‐lysosome transition . Our previous Snx10 silencing experiments in mice have shown its essential role in osteoclast vesicle trafficking and osteoclastic bone resorption . More recently, using yeast two‐hybrid screening and in vitro analyses, we have reported FKBP12 (an FK506 binding protein) as a partner for Snx10 for vesicular traffic in osteoclasts …”

The cast of clasts: catabolism and vascular invasion during bone growth, repair, and disease by osteoclasts, chondroclasts, and septoclasts

“…Besides these growth factors and their signaling pathways, there are essential transcription, signaling, and effector proteins necessary for osteoclast differentiation and activity which have been reviewed in some detail elsewhere (29, 30, 46, 47). Among the transcription factors are c-fos, PU.1, NFATc1, Mitf, and NF-κB.…”

Osteopetroses, emphasizing potential approaches to treatment

“…The protein is also expressed in the stomach, where high pH and low calcium solubilization are observed (22). Consequently, the SNX10 global knock-out mouse bone phenotype is consistent with osteopetrorickets, as opposed to the osteoclast-specific knock-out mice that show ostepetrosis but not rickets (21).…”

“…Furthermore, in other forms (gene implicated TCIRG1), patients also display increase of gastric pH, which may limit calcium absorption and contribute to hypocalcemia (19)(20). Based on data in mice (21), a similar increase of gastric pH could be hypothesized in patients harboring SNX10 mutations.…”