WHIM Syndrome-linked CXCR4 mutations drive osteoporosis.

WHIM Syndrome is a rare immunodeficiency caused by gain-of-function CXCR4 mutations. Here we report a decrease in bone mineral density in 25% of WHIM patients and bone defects leading to osteoporosis in a WHIM mouse model. Imbalanced bone tissue is observed in mutant mice combining reduced osteoprogenitor cells and increased osteoclast numbers. Mechanistically, impaired CXCR4 desensitization disrupts cell cycle progression and osteogenic commitment of skeletal stromal/stem cells, while increasing their pro-osteoclastogenic capacities. Impaired osteogenic differentiation is evidenced in primary bone marrow stromal cells from WHIM patients. In mice, chronic treatment with the CXCR4 antagonist AMD3100 normalizes in vitro osteogenic fate of mutant skeletal stromal/stem cells and reverses in vivo the loss of skeletal cells, demonstrating that proper CXCR4 desensitization is required for the osteogenic specification of skeletal stromal/stem cells. Our study provides mechanistic insights into how CXCR4 signaling regulates the osteogenic fate of skeletal cells and the balance between bone formation and resorption.

Culture, Expansion and Differentiation of Mouse Bone-Derived Mesenchymal Stromal Cells.

Mesenchymal stem/stromal cells (MSCs) are multipotent adult cells that are present in several tissues including the bone marrow (BM), in which they can differentiate in a variety of cell types such as osteoblasts, chondrocytes and adipocytes. The isolation of MSCs has been carried out by many studies that aim to control their differentiation into cartilaginous and bone cells in vitro in order to use this technology in the repair of damaged tissues. Here we describe the minimum requirements and an efficient method for isolation, expansion of mouse bone-derived multipotent mesenchymal stromal cells and their differentiation into osteoblasts, responsible for the bone matrix synthesis and mineralization.