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Retinoic acid receptors are required for skeletal growth, matrix homeostasis and growth plate function in postnatal mouse.

ABSTRACT: The retinoic acid receptors alpha, beta and gamma (RARalpha, RARbeta and RARgamma) are nuclear hormone receptors that regulate fundamental processes during embryogenesis, but their roles in skeletal development and growth remain unclear. To study skeletal-specific RAR function, we created conditional mouse mutants deficient in RAR expression in cartilage. We find that mice deficient in RARalpha and RARgamma (or RARbeta and RARgamma) exhibit severe growth retardation obvious by about 3 weeks postnatally. Their growth plates are defective and, importantly, display a major drop in aggrecan expression and content. Mice deficient in RARalpha and RARbeta, however, are virtually normal, suggesting that RARgamma is essential. In good correlation, we find that RARgamma is the most strongly expressed RAR in mouse growth plate and its expression characterizes the proliferative and pre-hypertrophic zones where aggrecan is strongly expressed also. By being avascular, those zones lack endogenous retinoids as indicated by previous RARE reporter mice and our direct biochemical measurements and thus, RARgamma is likely to exert ligand-less repressor function. Indeed, our data indicate that: aggrecan production is enhanced by RARgamma over-expression in chondrocytes under retinoid-free culture conditions; production is further boosted by co-repressor Zac1 or pharmacologic agents that enhance RAR repressor function; and RAR/Zac1 function on aggrecan expression may involve Sox proteins. In sum, our data reveal that RARs, and RARgamma in particular, exert previously unappreciated roles in growth plate function and skeletal growth and regulate aggrecan expression and content. Since aggrecan is critical for growth plate function, its deficiency in RAR-mutant mice is likely to have contributed directly to their growth retardation.

SUBMITTER: Williams JA

PROVIDER: S-EPMC4085816 | biostudies-literature | 2009 Apr

REPOSITORIES: biostudies-literature

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Retinoic acid receptors are required for skeletal growth, matrix homeostasis and growth plate function in postnatal mouse.

Williams Julie A JA Kondo Naoki N Okabe Takahiro T Takeshita Nobuo N Pilchak Diane M DM Koyama Eiki E Ochiai Takanaga T Jensen Deborah D Chu Mon-Li ML Kane Maureen A MA Napoli Joseph L JL Enomoto-Iwamoto Motomi M Ghyselinck Norbert N Chambon Pierre P Pacifici Maurizio M Iwamoto Masahiro M

Developmental biology 20090203 2

The retinoic acid receptors alpha, beta and gamma (RARalpha, RARbeta and RARgamma) are nuclear hormone receptors that regulate fundamental processes during embryogenesis, but their roles in skeletal development and growth remain unclear. To study skeletal-specific RAR function, we created conditional mouse mutants deficient in RAR expression in cartilage. We find that mice deficient in RARalpha and RARgamma (or RARbeta and RARgamma) exhibit severe growth retardation obvious by about 3 weeks post ...[more]

PMID: 19389355

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Project description:The growth plate contains resting and proliferating chondrocytes in its upper zones (UGP) and maturing and hypertrophic chondrocytes in its lower zones (LGP), but the mechanisms by which it operates to sustain skeletal growth are not fully clear. Retinoid signaling was previously found to be nearly absent in UGP, but to be much stronger in LGP coincident with hypertrophy, extracellular matrix turnover and endochondral bone formation. To determine whether such distinct signaling levels and phenotypic events reflect different endogenous retinoid levels, the upper two-thirds and lower one-third of rabbit rib growth plates were microsurgically isolated and processed for ultrasensitive retinoid LC-tandem MS quantification. Indeed, the UGP samples contained only about a 0.6 nm concentration of all-trans-retinoic acid (atRA) that is the most active natural retinoid in tissues, whereas LGP samples contained nearly 3-fold higher atRA levels (about 1.8 nM). Perichondrium was quite rich in atRA (about 4.9 nM). Interestingly, the levels of retinol, the major but inactive atRA precursor, were similar in all tissues (1.1-1.6 μM), suggesting that the distinct atRA levels in UGP and LGP reflect different retinoid anabolic capacity. Indeed, RALDH2 and CRABP1 transcript levels were much higher in LGP than UGP samples. To determine the minimum effective atRA concentration, chondrogenic cells transfected with a retinoic acid response element (RARE)-luc reporter plasmid were treated with different concentrations of exogenous atRA (0-100 nM). About 3 nm atRA was needed to elicit appreciable RARE-luc reporter activity and to decrease proteoglycan synthesis and activity of an aggrecan enhancer reporter plasmid. In sum, the data indicate that (i) the endogenous levels of atRA are significantly higher in hypertrophic than upper zones of growth plate; (ii) such difference likely reflects distinct retinoid anabolic capacity; and (iii) importantly, atRA levels in hypertrophic portion are within effective ranges to elicit retinoid signaling and action, but those in upper zones are not.

Dataset Information

Retinoic acid receptors are required for skeletal growth, matrix homeostasis and growth plate function in postnatal mouse.

Publications

Retinoic acid receptors are required for skeletal growth, matrix homeostasis and growth plate function in postnatal mouse.

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