Opposite effects of leptin on bone metabolism: a dose-dependent balance related to energy intake and insulin-like growth factor-I pathway.

Published data describing leptin effects on bone are at variance with both positive and negative consequences reported. These findings are consistent with a bimodal threshold response to serum leptin levels. To test this theory, two groups of female rats (tail-suspended and unsuspended) were treated with ip leptin at two different doses or vehicle for 14 d. In tail-suspended rats, low-dose leptin compensated the decrease in serum leptin levels observed with suspension and was able to prevent the induced bone loss at both the trabecular and cortical level (assessed by three-dimensional microtomography). In contrast, high-dose leptin inhibited femoral bone growth and reduced bone mass by decreasing bone formation rate and increasing bone resorption in both tail-suspended and unsuspended groups. High- and low-dose leptin administration resulted in a reduced medullar adipocytic volume in all groups. High-dose leptin (but not low) induced a decrease in body-weight abdominal fat mass and serum IGF-I levels. Thus, the observed bone changes at high-dose leptin are at least partly mediated by a leptin-induced energy imbalance. In conclusion, a balance between negative and positive leptin effects on bone is dependent on a bimodal threshold that is triggered by leptin serum concentration. Also, the negative effects of high leptin levels are likely induced by reduced energy intake and related hormonal changes. The respective part of each pathway will be unraveled by additional studies.

Global amplification polymerase chain reaction reveals novel transitional stages during osteoprogenitor differentiation.

Mesenchymal stem cells give rise to osteoprogenitors that proliferate and differentiate into identifiable preosteoblasts, osteoblasts, bone lining cells and osteocytes. To identify and establish a molecular profile for the more primitive and uncharacterized cells in the lineage, relatively rare (<1%) osteoprogenitors present in primary cultures of fetal rat calvaria cell populations were identified by a replica plating technique. Since the cell number was limited in each colony sampled, we used global amplification PCR to analyze the repertoire of genes expressed in osteoprogenitors. We established a molecular fingerprint and a developmental sequence based on simultaneous expression patterns for both known osteoblast-associated markers (collagen type I, alkaline phosphatase, osteopontin, bone sialoprotein, PTH1R and osteocalcin) and potential regulatory molecules (i.e. FGFR1, PDGF-Ralpha and PTHrP). By analysis of 99 osteoprogenitor and osteoblast colonies captured by replica plating at different developmental stages, we found: (1) a recognizable cohort of cells considered more primitive than committed osteoprogenitors; (2) a cohort of early progenitors transiently expressing bone sialoprotein; and (3) that mRNAs for FGF-R1, PDGF-Ralpha and PTH1R were expressed earlier than other markers and tended to increase and decrease in relative concert with the osteoblast-specific markers. The observations suggest that within the osteoblast differentiation sequence both discrete stages and continua of changing marker expression levels occur with variation in expression for any given marker. This combined approach of replica plating and global amplification PCR allows molecular fingerprinting of definitive primitive osteoprogenitors and will aid in identifying novel developmental stages and novel differentiation stage-specific genes as these cells progress through their differentiation sequence.