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Rachel A Davey, Michele V Clarke, Suzanne B Golub, Patricia K Russell, and Jeffrey D Zajac

The physiological role of calcitonin, and its receptor, the CTR (or Calcr), has long been debated. We previously provided the first evidence for a physiological role of the CTR to limit maternal bone loss during lactation in mice by a direct action on osteocytes to inhibit osteocytic osteolysis. We now extend these findings to show that CTR gene expression is upregulated two- to three-fold in whole bone of control mice at the end of pregnancy (E18) and lactation (P21) compared to virgin controls. This was associated with an increase in osteoclast activity evidenced by increases in osteoclast surface/bone surface and Dcstamp gene expression. To investigate the mechanism by which the CTR inhibits osteocytic osteolysis, in vivo acidification of the osteocyte lacunae during lactation (P14 days) was assessed using a pH indicator dye. A lower pH was observed in the osteocyte lacunae of lactating Global-CTRKOs compared to controls and was associated with an increase in the gene expression of ATPase H+ transporting V0 subunit D2 (Atp6v0d2) in whole bone of Global-CTRKOs at the end of lacation (P21). To determine whether the CTR is required for the replacement of mineral within the lacunae post-lactation, lacunar area was determined 3 weeks post-weaning. Comparison of the largest 20% of lacunae by area did not differ between Global-CTRKOs and controls post-lactation. These results provide evidence for CTR activation to inhibit osteocytic osteolysis during lactation being mediated by regulating the acidity of the lacunae microenvironment, whilst the CTR is dispensable for replacement of bone mineral within lacunae by osteocytes post-lactation.

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Patricia K Russell, Salvatore Mangiafico, Barbara C Fam, Michele V Clarke, Evelyn S Marin, Sofianos Andrikopoulos, Kristine M Wiren, Jeffrey D Zajac, and Rachel A Davey

It is well established that testosterone negatively regulates fat mass in humans and mice; however, the mechanism by which testosterone exerts these effects is poorly understood. We and others have shown that deletion of the androgen receptor (AR) in male mice results in a phenotype that mimics the three key clinical aspects of hypogonadism in human males; increased fat mass and decreased bone and muscle mass. We now show that replacement of the Ar gene specifically in mesenchymal progenitor cells (PCs) residing in the bone marrow of Global-ARKO mice, in the absence of the AR in all other tissues (PC-AR Gene Replacements), completely attenuates their increased fat accumulation. Inguinal subcutaneous white adipose tissue and intra-abdominal retroperitoneal visceral adipose tissue depots in PC-AR Gene Replacement mice were 50–80% lower than wild-type (WT) and 75–90% lower than Global-ARKO controls at 12 weeks of age. The marked decrease in subcutaneous and visceral fat mass in PC-AR Gene Replacements was associated with an increase in the number of small adipocytes and a healthier metabolic profile compared to WT controls, characterised by normal serum leptin and elevated serum adiponectin levels. Euglycaemic/hyperinsulinaemic clamp studies reveal that the PC-AR Gene Replacement mice have improved whole-body insulin sensitivity with higher glucose infusion rates compared to WT mice and increased glucose uptake into subcutaneous and intra-abdominal fat. In conclusion, these data provide the first evidence for an action of androgens via the AR in mesenchymal bone marrow PCs to negatively regulate fat mass and improve metabolic function.