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Osteocytes and calcium homeostasis


Challenging the currently held belief that only osteoclasts are able to remove bone matrix, Qing et al. show that osteocytes from lactating mice express genes such as tartrate resistant acid phosphatase (TRAP) and cathepsin K, which are usually only turned on in osteoclasts that are removing bone from the skeleton. Expression of these genes fell back to normal negligible levels when the animals weaned their offspring. These events were under the control of parathyroid hormone receptor 1 (PTHR1) signaling.

Conditions such as hyperparathyroidism, in which parathyroid hormone receptor protein or parathryoid hormone are produced at high levels are characterised by non-physiological bone remodeling by osteocytes. The observation that mice that lacked PTHR1 did not show any elevation of expression of these genes during lactation, and showed no remodeling of the perilacunar matrix, therefore suggests potential therapeutic implications.

Editor's comment: Although evidence has been growing that osteocytes are much more active than previously believed, this study is important because it demonstrates eloquently that osteocytes contribute directly to the regulation of calcium homeostasis. The finding that osteocytes can remove the perilacunar mineral during lactation and then replace it afterwards shows that osteocytes are able to recapitulate some aspects of the osteoclast phenotype.

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