Dental and craniofacial defects in the Crtap-/- mouse model of osteogenesis imperfecta type VII.

BACKGROUND: Inactivating mutations in the gene for cartilage-associated protein (CRTAP) cause osteogenesis imperfecta type VII in humans, with a phenotype that can include craniofacial defects. Dental and craniofacial manifestations have not been a focus of case reports to date. We analyzed the craniofacial and dental phenotype of Crtap-/- mice by skull measurements, micro-computed tomography (micro-CT), histology, and immunohistochemistry. RESULTS: Crtap-/- mice exhibited a brachycephalic skull shape with fusion of the nasofrontal suture and facial bones, resulting in mid-face retrusion and a class III dental malocclusion. Loss of CRTAP also resulted in decreased dentin volume and decreased cellular cementum volume, though acellular cementum thickness was increased. Periodontal dysfunction was revealed by decreased alveolar bone volume and mineral density, increased periodontal ligament (PDL) space, ectopic calcification within the PDL, bone-tooth ankylosis, altered immunostaining of extracellular matrix proteins in bone and PDL, increased pSMAD5, and more numerous osteoclasts on alveolar bone surfaces. CONCLUSIONS: Crtap-/- mice serve as a useful model of the dental and craniofacial abnormalities seen in individuals with osteogenesis imperfecta type VII.

Counter-regulatory phosphatases TNAP and NPP1 temporally regulate tooth root cementogenesis.

Cementum is critical for anchoring the insertion of periodontal ligament fibers to the tooth root. Several aspects of cementogenesis remain unclear, including differences between acellular cementum and cellular cementum, and between cementum and bone. Biomineralization is regulated by the ratio of inorganic phosphate (Pi) to mineral inhibitor pyrophosphate (PPi), where local Pi and PPi concentrations are controlled by phosphatases including tissue-nonspecific alkaline phosphatase (TNAP) and ectonucleotide pyrophosphatase/phosphodiesterase 1 (NPP1). The focus of this study was to define the roles of these phosphatases in cementogenesis. TNAP was associated with earliest cementoblasts near forming acellular and cellular cementum. With loss of TNAP in the Alpl null mouse, acellular cementum was inhibited, while cellular cementum production increased, albeit as hypomineralized cementoid. In contrast, NPP1 was detected in cementoblasts after acellular cementum formation, and at low levels around cellular cementum. Loss of NPP1 in the Enpp1 null mouse increased acellular cementum, with little effect on cellular cementum. Developmental patterns were recapitulated in a mouse model for acellular cementum regeneration, with early TNAP expression and later NPP1 expression. In vitro, cementoblasts expressed Alpl gene/protein early, whereas Enpp1 gene/protein expression was significantly induced only under mineralization conditions. These patterns were confirmed in human teeth, including widespread TNAP, and NPP1 restricted to cementoblasts lining acellular cementum. These studies suggest that early TNAP expression creates a low PPi environment promoting acellular cementum initiation, while later NPP1 expression increases PPi, restricting acellular cementum apposition. Alterations in PPi have little effect on cellular cementum formation, though matrix mineralization is affected.

MT1-MMP and type II collagen specify skeletal stem cells and their bone and cartilage progeny.

Skeletal formation is dependent on timely recruitment of skeletal stem cells and their ensuing synthesis and remodeling of the major fibrillar collagens, type I collagen and type II collagen, in bone and cartilage tissues during development and postnatal growth. Loss of the major collagenolytic activity associated with the membrane-type 1 matrix metalloproteinase (MT1-MMP) results in disrupted skeletal development and growth in both cartilage and bone, where MT1-MMP is required for pericellular collagen dissolution. We show here that reconstitution of MT1-MMP activity in the type II collagen-expressing cells of the skeleton rescues not only diminished chondrocyte proliferation, but surprisingly, also results in amelioration of the severe skeletal dysplasia associated with MT1-MMP deficiency through enhanced bone formation. Consistent with this increased bone formation, type II collagen was identified in bone cells and skeletal stem/progenitor cells of wildtype mice. Moreover, bone marrow stromal cells isolated from mice expressing MT1-MMP under the control of the type II collagen promoter in an MT1-MMP-deficient background showed enhanced bone formation in vitro and in vivo compared with cells derived from nontransgenic MT1-MMP-deficient littermates. These observations show that type II collagen is not stringently confined to the chondrocyte but is expressed in skeletal stem/progenitor cells (able to regenerate bone, cartilage, myelosupportive stroma, marrow adipocytes) and in the chondrogenic and osteogenic lineage progeny where collagenolytic activity is a requisite for proper cell and tissue function.

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.