Project Summary: Aging is associated with a decrease in bone mass that coincides with decreased numbers of early B cells (proB and preB cells). PreB cells approach the endosteum where bone forming osteoblasts and bone resorbing osteoclasts reside. PreB cells express two genes, IGLL1 and VPREB, whose proteins (λ5 and VpreB) form the surrogate light chain (SLC) that binds immunoglobulin µHC to form the cell-surface expressed preB cell receptor (preBCR). In both mice and humans, early B cell numbers and their λ5-expression decrease with age. Little is known about the role of early B cells and λ5 expression in bone homeostasis and aging. This represents a significant gap in our knowledge. Previously, we observed increased bone fragility of λ5-deficient mice. Since it is recognized that both early B cell numbers and λ5 expression normally decline with age, we reasoned that λ5 might contribute to skeletal aging. Our preliminary data show that absence of λ5 disrupts acquisition of trabecular bone, decreases trabecular bone mass and accelerates bone aging. This phenotype is more prominent in female than male mice, resembling the natural history of osteoporosis in human, and importantly this process can be reversed. Irradiated λ5-/- mice reconstituted with wild type (WT) bone marrow show recovery of trabecular bone mass, whereas loss of trabecular bone mass was observed when WT mice received λ5-/- marrow. We have identified a subset of mesenchymal stromal/stem cells (MSCs) that express λ5 protein. Similar to the aging- associated decrease in early B cell numbers, the numbers of λ5-expressing MSCs decrease with age. Our preliminary studies show that disrupting λ5, or its known ligands (heparan sulfate and galectins) in bone marrow MSC cultures, diminishes osteoblast development and function. Moreover, loss of λ5 in MSCs promotes their development into adipocytes at the expanse of osteoblasts, resulting in an MSC aging phenotype which induces increased adiposity. This suggests a role for λ5 in MSC developmental fate decision. Building on our novel findings, we will test the hypothesis that aging induces changes in λ5 expression in MSCs and early B cells that impact osteoblast development and function and lead to age-associated bone loss. Our premise is that studies using λ5-deficient (λ5-/-) and B cell-depleted (JH-/-) mice will elucidate mechanisms of λ5 control of bone homeostasis and skeletal aging. In specific Aim 1, we will determine how changes in λ5 expression affect skeletal aging. In aim 2, we will identify contributions of major cellular source(s) of λ5 to bone development and aging. In aim 3, we will determine the mechanisms by which λ5 controls bone homeostasis and aging. Our experimental approach incorporates the use of primary MSCs and early B cell co-culture, Mass Spec of early B cell secreted factors affecting MSCs, and adoptive transfer and conditional deletion to dissect the in-vivo role of λ5 during aging. The significance of this project...