Biglycan deficiency causes spontaneous aortic dissection and rupture in mice.

BACKGROUND: For the majority of cases, the cause of spontaneous aortic dissection and rupture is unknown. An inherited risk is associated with Marfan syndrome, Ehlers-Danlos syndrome type IV, and loci mapped to diverse autosomal chromosomes. Analysis of pedigrees however has indicated that it may be also inherited as an X-linked trait. The biglycan gene, found on chromosome X in humans and mice, encodes a small leucine-rich proteoglycan involved in the integrity of the extracellular matrix. A vascular phenotype has never been described in mice deficient in the gene for small leucine-rich proteoglycans. In the breeding of BALB/cA mice homozygous for a null mutation of the biglycan gene, we observed that 50% of biglycan-deficient male mice died suddenly within the first 3 months of life. METHODS AND RESULTS: Necropsies revealed a major hemorrhage in the thoracic or abdominal cavity, and histology showed aortic rupture that involved an intimal and medial tear as well as dissection between the media and adventitia. By transmission electron microscopy and biomechanical testing, the aortas of biglycan-deficient mice showed structural abnormalities of collagen fibrils and reduced tensile strength. Similar collagen fibril changes were observed in male as well as in female biglycan-deficient mice, which implies a role of additional determinants such as gender-related response to stress in the development of this vascular catastrophe only in male mice. CONCLUSIONS: The spontaneous death of biglycan-deficient male mice from aortic rupture implicates biglycan as essential for the structural and functional integrity of the aortic wall and suggests a potential role of biglycan gene defects in the pathogenesis of aortic dissection and rupture in humans.

Biglycan deficiency increases osteoclast differentiation and activity due to defective osteoblasts.

Bone mass is maintained by a fine balance between bone formation by osteoblasts and bone resorption by osteoclasts. Although osteoblasts and osteoclasts have different developmental origins, it is generally believed that the differentiation, function, and survival of osteoclasts are regulated by osteogenic cells. We have previously shown that the extracellular matrix protein, biglycan (Bgn), plays an important role in the differentiation of osteoblast precursors. In this paper, we showed that Bgn is involved in regulating osteoclast differentiation through its effect on osteoblasts and their precursors using both in vivo and in vitro experiments. The in vivo osteolysis experiment showed that LPS (lipopolisaccharide)-induced osteolysis occurred more rapidly and extensively in bgn deficient mice compared to wild type (WT) mice. To further understand the mechanism of action, we determined the effects of Bgn on 1alpha, 25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3))-induced osteoclast differentiation and bone resorption in an co-culture of calvariae-derived pre-osteoblasts and osteoclast precursors derived from spleen or bone marrow. Time course and dose response experiments showed that tartrate-resistant acid phosphatase-positive multinuclear cells appeared earlier and more extensively in the co-cultures containing calvarial cells from bgn deficient mice than WT mice, regardless of the genotype of osteoclast precursors. The osteoblast abnormality that stimulated osteoclast formation appeared to be independent of the differential production of soluble RANKL and OPG and, instead, due to a decrease in osteoblast maturation accompanied by increase in osteoblastic proliferation. In addition to the imbalance between differentiation and proliferation, there was a differential decrease in secretory leukocyte protease inhibitor (slpi) in bgn deficient osteoblasts treated with 1,25-(OH)(2)D(3). These findings point to a novel molecular factor made by osteoblasts that could potentially be involved in LPS-induced osteolysis.

Biglycan deficiency interferes with ovariectomy-induced bone loss.

UNLABELLED: Biglycan is a matrix proteoglycan with a possible role in bone turnover. In a 4-week study with sham-operated or OVX biglycan-deficient or wildtype mice, we show that biglycan-deficient mice are resistant to OVX-induced trabecular bone loss and that there is a gender difference in the response to biglycan deficiency. INTRODUCTION: Biglycan (bgn) is a small extracellular matrix proteoglycan enriched in skeletal tissues, and biglycan-deficient male mice have decreased trabecular bone mass and bone strength. The purpose of this study was to investigate the bone phenotype of the biglycan-deficient female mice and to investigate the effect of estrogen depletion by ovariectomy (OVX). MATERIALS AND METHODS: OVX or sham operations were performed on 21-week-old mice that were divided into four groups: wt sham (n = 7), wt OVX (n = 9), bgn-deficient sham (n = 10) and bgn-deficient OVX (n = 10). The mice were killed 4 weeks after surgery. Bone mass and bone turnover were analyzed by peripheral quantitative computed tomography (pQCT), biochemical markers, and histomorphometry. RESULTS AND CONCLUSIONS: In contrast to the male mice, there were only few effects of bgn deficiency on bone metabolism in female mice, showing a clear gender difference. However, when stressed by OVX, the female bgn knockout (KO) mice were resistant to the OVX-induced trabecular bone loss. The wt mice showed a decrease in trabecular bone mineral density by pQCT measurements, a decrease in trabecular bone volume (BV/TV), and an increase in mineral apposition rate. In contrast, no significant changes were detected in bgn KO mice after OVX. In addition, analysis of the bone resorption marker deoxypyridinoline showed no significant increase in the bgn KO OVX mice compared with bgn KO sham mice. Measurements of serum osteoprotegerin (OPG) and RANKL revealed increased levels of OPG and decreased levels of RANKL in the bgn KO mice compared with wt mice. In conclusion, the bgn deficiency protects against increased trabecular bone turnover and bone loss in response to estrogen depletion, supporting the concept that bgn has dual roles in bone, where it may modulate both formation and resorption ultimately influencing the bone turnover process.

Transforming growth factor-beta controls human osteoclastogenesis through the p38 MAPK and regulation of RANK expression.

Although RANK-L is essential for osteoclast formation, factors such as transforming growth factor-beta (TGF-beta) are potent modulators of osteoclastogenic stimuli. To systematically investigate the role of TGF-beta in human osteoclastogenesis, monocytes were isolated from peripheral blood by three distinct approaches, resulting in either a lymphocyte-rich, a lymphocyte-poor, or a pure osteoclast precursor (CD14-positive) cell population. In each of these osteoclast precursor populations, the effect of TGF-beta on proliferation, TRAP activity, and bone resorption was investigated with respect to time and length of exposure. When using the highly pure CD14 osteoclast precursor cell population, the effect of TGF-beta was strongly dependent on the stage of osteoclast maturation. When monocytes were exposed to TGF-beta during the initial culture period (days 1-7), TRAP activity and bone resorption were increased by 40%, whereas the cell number was reduced by 25%. A similar decrease in cell number was observed when TGF-beta was present during the entire culture period (days 1-21), but in direct contrast, TRAP activity, cell fusion, cathepsin K, and matrix metalloproteinase (MMP)-9 expression as well as bone resorption were almost completely abrogated. Moreover, we found that latent TGF-beta was strongly activated by incubation with MMP-9 and suggest this to be a highly relevant mechanism for regulating osteoclast activity. To further investigate the molecular mechanism responsible for the divergent effects of continuous versus discontinuous exposure to TGF-beta, we examined RANK expression and p38 MAPK activation. We found the TGF-beta strongly induced p38 MAPK in monocytes, but not in mature osteoclasts, and that continuous exposure of TGF-beta to monocytes down-regulated RANK expression. The current results suggest that TGF-beta promotes human osteoclastogenesis in monocytes through stimulation of the p38 MAPK, whereas continuous exposure to TGF-beta abrogates osteoclastogenesis through down-regulation of RANK expression and therefore attenuation of RANK-RANK-L signaling.