Mouse model of secondary hyperparathyroidism results in osteocyte-mediated reduction of long-term hematopoietic stem cells

Presentation Number: OR43-3
Date of Presentation: June 18th, 2013

Benjamin J. Frisch*1, Xiaolin Tu2, Olga Bromberg1, Alexandra Goodman1, Teresita Bellido2 and Laura Maria Calvi1
1University of Rochester School of Medicine, Rochester, NY, 2Indiana University, Indianapolis, IN

Abstract

Abundant data support the central role of osteolineage cells in regulation of Hematopoietic stem cells (HSCs).  We previously demonstrated that intermittent treatment with parathyroid hormone, PTH(1-34) increases HSCs in mice through an indirect mechanism, as HSCs do not express the PTH receptor (PTH1R). In contrast, the effect of secondary hyperparathyroidism on marrow HSCs is unknown. We analyzed marrow HSC numbers in a mouse model of secondary hyperparathyroidism. Mice were fed a fed a Normal (N) diet (0.97% calcium) or low-calcium (low Ca) diet (0.01% calcium) for 14 days, which has previously been demonstrated to cause secondary hyperparathyroidism in as early as 7 days.   Flow cytometric analysis of the marrow from mice fed low Ca diet for 14 days demonstrated significant reduction in phenotypic long-term HSCs (LT-HSCs) as compared to controls (0.0012 ± 0.00012 vs 0.0006 ± 0.00012 % N vs Low Ca diet p≤0.01 N=7 mice/group). In the Low Ca diet group, short-term HSCs (ST-HSCs) in the marrow and the peripheral blood cell counts of white blood cells, platelets or hematocrit were unchanged, suggesting an effect of the low Ca diet specifically on LT-HSCs. Osteocytes have been implicated as critical hormonal targets and mediators of PTH action on bone.  To delineate if the increase in PTH and its action through a population of osteolineage cells mediate the LT-HSCs decrease in the low Ca diet subjects, we utilized mice lacking the PTH1R in osteocytes (OCy-PTH1Rko), which were exposed to either N or Low Ca diet for 14 days.  As expected, in the low Ca diet, PTH levels were increased above mean normal by 7 days in both wt and OCy-PTH1Rko, suggesting that lack of the PTH1R in osteocytes does not affect the development of secondary hyperparathyroidism. Baseline bone marrow cellularity and LT-HSCs were normal in OCy-PTH1Rko on N diet.  However, the decline in LT-HSCs demonstrated in wildtype mice in the setting of a Low Ca diet was blocked in OCy-PTH1Rko mice. Our findings demonstrate a differential effect on HSCs resulting from pharmacologic PTH (1-34) treatment vs PTH elevation due to secondary hyperparathyroidism. These data also suggest that continuous exposure of osteocytes to PTH decreases marrow microenvironmental support of LT-HSCs, consistent with our previous work using a genetic mouse model targeting a constitutively active PTH1R receptor to osteocytes.  These data also confirm the differential roles of osteolineage cell subsets in the HSC niche, and suggest that secondary hyperparathyroidism perturbs the HSC niche.

 

Nothing to Disclose: BJF, XT, OB, AG, TB, LMC