LRP6 in mesenchymal stem cells is required for bone formation during bone growth and bone remodeling

Abstract

Lipoprotein receptor-related protein 6 (LRP6) plays a critical role in skeletal development and homeostasis in adults. However, the role of LRP6 in mesenchymal stem cells (MSCs), skeletal stem cells that give rise to osteoblastic lineage, is unknown. In this study, we generated mice lacking LRP6 expression specifically in nestin⁺ MSCs by crossing nestin-Cre mice with LRP6 ^flox mice and investigated the functional changes of bone marrow MSCs and skeletal alterations. Mice with LRP6 deletion in nestin⁺ cells demonstrated reductions in body weight and body length at 1 and 3 months of age. Bone architecture measured by microCT (µCT) showed a significant reduction in bone mass in both trabecular and cortical bone of homozygous and heterozygous LRP6 mutant mice. A dramatic reduction in the numbers of osteoblasts but much less significant reduction in the numbers of osteoclasts was observed in the mutant mice. Osterix⁺ osteoprogenitors and osteocalcin⁺ osteoblasts significantly reduced at the secondary spongiosa area, but only moderately decreased at the primary spongiosa area in mutant mice. Bone marrow MSCs from the mutant mice showed decreased colony forming, cell viability and cell proliferation. Thus, LRP6 in bone marrow MSCs is essential for their survival and proliferation, and therefore, is a key positive regulator for bone formation during skeletal growth and remodeling.

Bone formation: Controlling bone stem cells

A cell surface protein involved in bone formation is required for cell division and survival of skeletal stem cells. Lipoprotein receptor-related protein 6 (LRP6) is known to play an important role in the development and maintenance of mature bone cells but its role in skeletal stem cells had been less clear. A group led by Mei Wan at Johns Hopkins University School of Medicine in Baltimore, Maryland, USA, showed that mice that lacked LRP6 in skeletal stem cells were lighter and smaller than normal mice, and their bones were thinner. They had fewer mature bone cells, and their stem cells showed reduced survival and proliferation when compared with stem cells from normal mice. The findings suggest that LRP6 is important for bone growth during development, and bone maintenance during adulthood.

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