Nmp4/CIZ suppresses parathyroid hormone‐induced increases in trabecular bone

Abstract

The nucleocytoplasmic shuttling transcription factor Nmp4/CIZ (nuclear matrix protein 4/cas interacting zinc finger protein) is a ubiquitously expressed protein that regulates both cytoplasmic and nuclear activities. In the nucleus, Nmp4/CIZ represses transcription of genes crucial to osteoblast differentiation and genes activated by various anabolic stimuli, including parathyroid hormone (PTH). We investigated the role of Nmp4/CIZ in the PTH-induced increase in bone by engineering mice with loss-of-function mutations in the Nmp4/CIZ gene, and treating 10-week-old female mice with anabolic doses of human PTH (1–34) at 30 μg/kg/day, 7 day/week, for 7 weeks or vehicle control. The untreated, baseline phenotype of the Nmp4-null mice between 8 and 16 weeks of age included a modest but significant increase in bone mineral density (BMD) and bone mineral content (BMC) compared to wild-type (WT) mice. Type I collagen mRNA expression was moderately elevated in the femurs of the Nmp4-null mice. The Nmp4 mutant alleles decreased body weight by 4% when expressed on a mixed background but the same alleles on a pure B6 background yielded a significant, 15% increase in body weight among the KO mice, compared to their WT controls. Hormone treatment equally enhanced BMD and BMC over vehicle-treated mice in both the WT and Nmp4-null groups but Nmp4-KO mice exhibited a significantly greater PTH-induced acquisition of femoral trabecular bone as compared to WT mice. These data support our hypothesis that Nmp4/CIZ is a transcriptional attenuator that suppresses osteoid synthesis and PTH-mediated acquisition of cancellous bone.

Nmp4/CIZ Suppresses the Parathyroid Hormone Anabolic Window by Restricting Mesenchymal Stem Cell and Osteoprogenitor Frequency

“…Although these transcription factors mediate numerous functions within the context of bone, this is consistent with an enhanced PTH-induced increase in mesenchymal stem cell self-renewal and/or recruitment of null osteoblasts and osteoclasts into the anabolic window [9][10][11][12][13]. Interestingly, the untreated Nmp4-KO mice had a modest, but significantly elevated, bone mineral density and bone mineral content as compared to WT animals [8] despite modestly elevated levels of serum C-terminal telopeptides of type I collagen (CTX), a marker for bone resorption [7]. The Nmp4-KO bone marrow (BM) yielded *1.8-fold more osteoclasts in vitro as compared to WT marrow, and the null osteoclasts were significantly more active than their WT counterparts [7].…”

“…However, in the null mice, this PTH-induced surge in serum osteocalcin exceeded that observed in the WT mice and was still climbing at the end of the 7-week treatment regimen [7]. Consistent with this sustained serum osteocalcin surge, at the end of the 7-week treatment period, the null mice had gained significantly more femoral, vertebral, and tibial trabecular bone than WT mice while maintaining robust increases in cortical bone [7,8]. These enhanced increases in cancellous bone in the Nmp4-KO skeleton all showed significant treatment · genotype interactions, thus demonstrating that Nmp4/CIZ suppresses PTH-stimulated anabolism [7,8].…”

“…We previously reported no differences in the femur length between Nmp4-KO and WT mice at 8 and 17 weeks of age [8]. Therefore, after euthanasia, a 2.6-mm span (*5 mm 3 of medullary space) of the distal femoral metaphysis was scanned in 70% ethanol on a desktop microcomputed tomography (mCT) (mCT 35; Scanco Medical AG) at 10-mm resolution using 55-kVp tube potential and 400-ms integration time to measure trabecular three-dimensional (3D) morphometric properties as previously described [17].…”

“…Nmp4-KO mice, backcrossed onto a C57BL/6J background for 6-7 generations, [7,8], and their WT littermates were used for these studies. Our local Institutional Animal Care and Use Committee approved all experiments and procedures involving the production and use of the mice described in this investigation.…”

“…While the mechanisms regulating the extent of the PTH anabolic window are unknown, we demonstrated that disabling the transcription factor nuclear matrix protein 4/ cas-interacting zinc-finger protein (Nmp4/CIZ) in mice significantly extends and augments PTH bone-forming capacity; treatment of wild-type (WT) and Nmp4-knockout (KO) mice with intermittent PTH for 7 weeks resulted in significant increases in serum osteocalcin, a marker for bone formation, but these serum profiles as a function of time were strikingly different [7,8]. In the WT mice, serum osteocalcin peaked at 3 weeks of treatment and returned to baseline by 7 weeks of hormone administration [7].…”