A potential role for the myeloid lineage in leptin-regulated bone metabolism

Abstract

Leptin influences bone formation centrally through the hypothalamus and peripherally by acting on osteoblasts or their precursors. However, neither mechanism explains the divergent, gender-specific correlation between leptin and bone mineral density in humans. Although leptin is a potent regulator of pro-inflammatory immune responses, a potential role for leptin as an osteoimmunologic intermediate in bone metabolism has not been tested. Mice with myeloid-specific ablation of the long-form leptin receptor (ObRb) were generated using mice expressing cre-recombinase from the lysoszyme M promoter. At 12 weeks of age, the conditional knockout mice did not display any appreciable phenotype. However, at 52 weeks 2 changes were noted. First, there was a mild increase in liver inflammation. Second, a gender-specific, divergent bone phenotype was observed. Female mice displayed a consistent trend toward decreased trabecular bone parameters including reductions in bone volume fraction, trabecular number, and bone mineral content as well as a significant increase in marrow adipogenesis. Conversely, male mice lacked trabecular changes, but had statistically significant increases in cortical bone volume, thickness, and bone mineral density with equivalent total cortical volume. Since the year 2000, over 25 studies on more than 10,000 patients have sought to determine the correlation between leptin and bone mineral density. The results revealed a gender-specific correlation similar to that observed in our LysM transgenic animals. We hypothesize and show new evidence that regulation of myeloid lineage cells by leptin may facilitate their actions as an osteoimmunologic intermediate and contribute to leptin-regulated bone formation and metabolism in a gender-specific manner.

Figure 1. Generation of LysM^Cre+F/F Mice

LysM-Cre mice were mated with mice with loxP sites flanking exon 17 of the long-form leptin receptor (ObRb) to generate myeloid-specific ObRb knock-out mice. (A) Genomic DNA PCR of tissues from 12 week old animals or primary bone marrow macrophages cultured in vitro showing the intact (~646 base pairs) and deleted (~200 base pairs) bands, lanes cropped from the same gel. WBM = Whole Bone Marrow. (B) Representative photo, female, 3 months old. (C) Body mass of transgenic female mice from 3 to 52 weeks (N=6–8). (D) 52 week old female liver mass and liver mass/body mass ratio (N=6–8). (E) Liver histology. (F) Number of inflammatory islands per field (N=3 mice, 3 liver sections each, 10 fields/section). Inset contains genomic DNA PCR to check for loxP recombination. (G) Tibial bone marrow histology, 10× magnification. Numbers represent adipocyte number per marrow area (mm2) plus or minus the standard deviation (N=4–7). a: significant compared to Cre- female; b: significant over Cre- male; c: significant over Cre+ male.