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Dose‐Dependent Effects of Runx2 on Bone Development
Author(s) -
Zhang Shiqin,
Xiao Zhousheng,
Luo Junming,
He Nan,
Mahlios Josh,
Quarles L Darryl
Publication year - 2009
Publication title -
journal of bone and mineral research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.882
H-Index - 241
eISSN - 1523-4681
pISSN - 0884-0431
DOI - 10.1359/jbmr.090502
Subject(s) - runx2 , chromatin immunoprecipitation , promoter , osteoblast , biology , microbiology and biotechnology , transcription factor , gene expression , gene , genetics , in vitro
Abstract Runx2 controls the commitment of mesenchymal cells to the osteoblastic lineage. Distinct promoters, designated P1 and P2, give rise to functionally similar Runx2 ‐II and Runx2 ‐I isoforms. We postulate that this dual promoter gene structure permits temporal and spatial adjustments in the amount of Runx2 isoforms necessary for optimal bone development. To evaluate the gene dose‐dependent effect of Runx2 isoforms on bone development, we intercrossed selective Runx2‐II +/− with nonselective Runx2‐II +/− /Runx2‐I +/− mice to create compound mutant mice: Runx2‐II +/− , Runx2‐II +/− /Runx2‐I +/− , Runx2‐II −/− , Runx2‐II −/− /Runx2‐I +/− , Runx2‐II −/− /Runx2‐I −/− . Analysis of the different Runx2‐deficient genotypes showed gene dose‐dependent differences in the level of expression of the Runx2 isoforms. In addition, we found that Runx2‐I is predominately expressed in the perichondrium and proliferating chondrocytes, whereas Runx2 ‐II is expressed in hypertrophic chondrocytes and metaphyseal osteoblasts. Newborn mice showed impaired development of a mineralized skeleton, bone length, and widening of the hypertrophic zone that were proportionate to the reduction in total Runx2 protein expression. Osteoblast differentiation ex vivo was also proportionate to total amount of Runx2 expression that correlated with reduced Runx2 binding to the osteocalcin promoter by quantitative chromatin immunoprecipitation analysis. Functional analysis of P1 and P2 promoters showed differential regulation of the two promoters in osteoblastic cell lines. These findings support the possibility that the total amount of Runx2 derived from two isoforms and the P1 and P2 promoters, by regulating the time, place, and amount of Runx2 in response to changing environmental cues, impacts on bone development.