Letter re: “The 3.6 kb DNA fragment from the rat Col1a1 gene promoter drives the expression of genes in both osteoblast and osteoclast lineage cells” by Boban et al. (Bone 39:1302–1312, 2006)

Abstract

In the paper by Boban and colleagues [1], the authors used a parabiosis mouse model to test whether osteoblast-lineage cells traversed the circulation of the parabiosed pair. Using mice expressing GFP driven by the col3.6 promoter, they found that there were GFP-positive cells lining bone surfaces in the non-GFP-expressing parabiosed mouse, indicating extensive transfer of GFP-positive cells. In addition, when they infused Lin − Sca-1 + c-kit + cells from a col3.6-GFP mouse into a non-GFP-expressing mouse, these cells also appeared on bone surfaces as GFP-positive lining cells. However, based on immunohistochemistry for TRAP and in vitro bone marrow cultures, the investigators concluded that the GFP-positive cells lining bone surfaces in the two experiments described above were osteoclasts (or osteoclastlineage cells), and that there was no transfer of osteoblast-lineage cells between the parabiotic pairs or differentiation of the Lin − Sca-1 + c-kit + cells into osteoblastic cells.There are some concerns with the interpretation of the findings, and the authors are perhaps overlooking another important cell type, the bone lining cell, which is likely closely related to the osteoblast, but probably serves a very distinct functional role [2]. The alternate explanation for the findings in this paper are that the GFP-positive cells that are lining the bone surfaces in the parabiosis model and in the mice infused with the Lin − Sca-1 + c-kit + cells are, in fact, bone lining cells. These cells lie in close proximity to osteoclasts [2], express bone-related proteins such as alkaline phosphatase, low levels of osteocalcin and type I collagen (i.e., they are not highly active in synthesizing matrix as are the osteoblasts on bone surfaces), and are also positive for expression of intercellular adhesion molecule-1 (ICAM-1) [2,3], which appears to be necessary for binding to osteoclast precursors and the subsequent support of osteoclastogenesis [4]. By contrast, matrix-synthesizing osteoblasts express high levels of type I collagen and are ICAM-1 negative [2]. Moreover, since bone lining cells lie adherent to bone surfaces, they are unlikely to be present in the bone marrow cultures used in the paper to support the argument that osteoblastic cells did not transfer in the parabiotic mice.

Col3.6-HSD2 transgenic mice: A glucocorticoid loss-of-function model spanning early and late osteoblast differentiation

“…The Col3.6 promoter has been reported to direct very low level expression of a GFP transgene in the osteoclast lineage [33], although some of these cells may be bone lining cells [34]. Some studies have reported that glucocorticoids exert direct effects on osteoclasts.…”