Osteopontin: a bridge between bone and blood.

The production of mature blood cells within the bone marrow (BM) is attributed to a pool of haemopoietic stem cells (HSC). It is now evident that HSC reside preferentially at the endosteal region within the BM where bone-lining osteoblasts are a key cellular component of the HSC niche that directly regulates HSC fate. Osteoblasts synthesise proteins that stimulate and inhibit HSC proliferation. In addition to angiopoietin 1 (Ang-1), osteoblasts synthesise and express the highly acidic glycoprotein, osteopontin (Opn), which, like Ang-1, acts as a potent constraining factor on HSC proliferation. Overexpression of Opn is a feature of haemopoietic malignancies, such as multiple myeloma and chronic myeloid leukaemia, although its exact role in the aetiology and progression of these diseases remains unclear. Through osteoblasts and their cell surface and expressed proteins including Opn, bone is able to regulate the tissue that resides within it. In doing so, Opn can be considered a bridge between bone and blood.