Cellular Mechanism of Decreased Bone in Brtl Mouse Model of OI: Imbalance of Decreased Osteoblast Function and Increased Osteoclasts and Their Precursors

Abstract

ABSTRACT:The Brtl mouse, a knock-in model for moderately severe osteogenesis imperfecta (OI), has a G349C substitution in half of type I collagen ␣1(I) chains. We studied the cellular contribution to Brtl bone properties. Brtl cortical and trabecular bone are reduced before and after puberty, with BV/TV decreased 40-45%. Brtl ObS/BS is comparable to wildtype, and Brtl and wildtype marrow generate equivalent number of colony-forming units (CFUs) at both ages. However, OcS/BS is increased in Brtl at both ages (36-45%), as are TRACP + cell numbers (57-47%). After puberty, Brtl ObS/BS decreases comparably to wildtype mice, but osteoblast matrix production (MAR) decreases to one half of wildtype values. In contrast, Brtl OcS falls only moderately (∼16%), and Brtl TRACP staining remains significantly elevated compared with wildtype. Consequently, Brtl BFR decreases from normal at 2 mo to one half of wildtype values at 6 mo. Immunohistochemistry and real-time RT-PCR show increased RANK, RANKL, and osteoprotegerin (OPG) levels in Brtl, although a normal RANKL/OPG ratio is maintained. TRACP + precursors are markedly elevated in Brtl marrow cultures and form more osteoclasts, suggesting that osteoclast increases arise from more RANK-expressing precursors. We conclude that osteoblasts and osteoclasts are unsynchronized in Brtl bone. This cellular imbalance results in declining BFR as Brtl ages, consistent with reduced femoral geometry. The disparity in cellular number and function results from poorly functioning osteoblasts in addition to increased RANK-expressing precursors that respond to normal RANKL/OPG ratios to generate more bone-resorbing osteoclasts. Interruption of the stimulus that increases osteoclast precursors may lead to novel OI therapies.

Immature Osteoblast Lineage Cells Increase Osteoclastogenesis in Osteogenesis Imperfecta Murine

“…Because OIM is marked by the block in osteoblast differentiation, accumulation of immature osteoblast lineage cells may induce changes in bone cellular microenvironment toward enhanced osteoclast differentiation. 19 Our experiments using TNF-␣ antibody showed a partial block of the osteoclastogenesis in the coculture system. Therefore, the higher number and enhanced resorptive activity of osteoclasts formed in coculture with oim/oim osteoblasts can be explained by the synergistic effect of TNF-␣, highly expressed by immature osteoblasts, along with Rankl and M-CSF also produced by osteoblasts.…”

“…25 Defective collagen present in osteogenesis imperfecta may also trigger the secretion of inflammatory cytokines. 19 This hypothesis is supported by the finding of approximate threefold increase of TNF-␣ expression in oim/oim osteoblasts compared with ϩ/ϩ osteoblasts. In addition, we also found that less mature GFP Ϫ populations in wild-type mice have increased TNF-␣ expression compared with corresponding GFP ϩ populations.…”

“…Although the success of the bisphosphonates in the treatment for the OI has been well documented and widely used, recent work by Uveges et al presented some of the potential negative effects of bisphosphonate treatment. 19 This underlines the importance for better understanding the complexity of the bone cell responses (osteoblast and osteoclast) to the formation of a defective matrix.…”

Near-infrared fluorescent probe traces bisphosphonate delivery and retention in vivo

“…(56) Conversely, high levels of bone turnover may play a role in clearing the skeleton of deposited bisphosphonate. Pamidronate has been detected in the urine of OI patients 8 years following cessation of treatment, (22) suggesting that despite local bisphosphonate concentration, drug still can be turned over in the body as osteoclasts attempt to remodel treated bone.…”

In utero transplantation of adult bone marrow decreases perinatal lethality and rescues the bone phenotype in the knockin murine model for classical, dominant osteogenesis imperfecta

“…The animals were genotyped by polymerase chain reaction (PCR) using genomic DNA from tail biopsies. 36 Tissue examination by fluorescence and confocal microscopy Spleen, thymus, liver, lung, brain, pancreas, kidney, fat, testis, and calvarial bone from killed mice were examined by DMIL-Leica fluorescence microscope (Leica). Mice containing eGFP ϩ cells were used for subsequent studies.…”