Diabetes Mellitus (DM) represents one of the major threats to human health. Our group recently demonstrated an impairment of bone marrow (BM) stem cell (SC) pool during DM and a peculiar remodeling of BM affecting the endosteal SC niche. Moreover osteoporosis is often associated with DM and osteoclast (OCL) activation has a crucial role in homeostasis and mobilization of BM SCs. The aim of this study is to investigate if DM increases OCL activity, thus leading to an altered release of SCs from BM into the circulation.Methods
Type-1 DM was induced in CD1 mice by streptozotocin (STZ, 40 mg/Kg/day IP for five consecutive days). Control mice received the STZ vehicle. DM induction was verified by assessment of glycosuria. Flow cytrometry was used to evaluate the SC abundance in peripheral blood (PB). Immunohistochemical analyses were performed to assess the BM structure and the expression of Tartrate Resistant Acid Phosphatase (TRAP), an indicator of OCL activation. Semi-Quantitative Real Time PCR was used to determine mRNA levels of Receptor Activator of Nuclear Factor Kappa-B Ligand (RANKL) an inducer of OCL differentiation and activation.Results
Flow cytrometry studies showed a remarkable increase in the fraction of lineage negative, Sca-1 positive and cKit positive (LSK) cells in PB of mice with early stage DM (5 weeks) compared with non-diabetic controls (0.2 vs. 0.7% of total MNCs respectively, p < 0.05). Analysis of femoral bones confirmed the progressive remodeling of BM with reduction of the hematopoietic component, fat accumulation and bone rarefaction. Likewise, the number of activated OCLs expressing TRAP increased in femoral bones from early to late stages of DM as compared with age-matched non-diabetic controls. At 20 weeks of DM, the expression of RANKL was found increased in cells isolated from the endosteal region. However, RANKL expression was reduced in total BM of diabetic mice compared to controls.Conclusion
Our data show that circulating SCs are increased at initial phase of DM in coincidence with activation of OCLs and before any apparent alteration of hematopoiesis. Continuous OCL activation is sustained in endosteal region by induction of RANKL, which may contribute to the detachment of SCs from endosteal niche and eventually cell depletion at late stage of DM. The OCL-induced remodeling of the endosteal niche, which contains the most primitive SCs in BM, may contribute to the impairment of regenerative capacity.