DNA damage and senescence in osteoprogenitors expressing Osx1 may cause their decrease with age

Abstract

SummaryAge‐related bone loss in mice results from a decrease in bone formation and an increase in cortical bone resorption. The former is accounted by a decrease in the number of postmitotic osteoblasts which synthesize the bone matrix and is thought to be the consequence of age‐dependent changes in mesenchymal osteoblast progenitors. However, there are no specific markers for these progenitors, and conclusions rely on results from in vitro cultures of mixed cell populations. Moreover, the culprits of such changes remain unknown. Here, we have used Osx1‐Cre;TdRFP mice in which osteoprogenitors express the TdRFP fluorescent protein. We report that the number of TdRFP‐Osx1 cells, freshly isolated from the bone marrow, declines by more than 50% between 6 and 24 months of age in both female and male mice. Moreover, TdRFP‐Osx1 cells from old mice exhibited markers of DNA damage and senescence, such as γH2AX foci, G1 cell cycle arrest, phosphorylation of p53, increased p21CIP 1 levels, as well as increased levels of GATA4 and activation of NF‐κB – two major stimulators of the senescence‐associated secretory phenotype (SASP). Bone marrow stromal cells from old mice also exhibited elevated expression of SASP genes, including several pro‐osteoclastogenic cytokines, and increased capacity to support osteoclast formation. These changes were greatly attenuated by the senolytic drug ABT263. Together, these findings suggest that the decline in bone mass with age is the result of intrinsic defects in osteoprogenitor cells, leading to decreased osteoblast numbers and increased support of osteoclast formation.

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“…The functional role of ROS in age-dependent bone loss has been demonstrated by attenuation of the age-related decline in femoral cortical thickness by overexpression of catalase in cells of the osteoblast lineage (23). Finally, other studies of ours suggest that senescence of osteoblast progenitors leading to cell cycle arrest increases with age, which may contribute to the reduction in the number of endosteal osteoblasts (69).…”

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“…Interestingly, this was due to arrest at the G1 phase of the cell cycle . Moreover, osteoprogenitors from old mice exhibited several characteristics of senescent cells, including elevated levels of γH2AX foci (a marker of DNA damage due to double‐strand breaks), phosphorylated p53, elevated p21 expression, and upregulation of several SASP factors such as GATA4 and activation of NF‐κB . Complementary to these findings in osteoprogenitors, Farr and colleagues found upregulation of several SASP factors with aging, predominantly in enriched populations of myeloid cells and osteocytes.…”

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“…(66,86–90) Last, targeted apoptosis of senescent cells with so-called senolytic agents restores tissue homeostasis in response to chemotoxicity and aging and attenuates atherosclerosis and posttraumatic osteoarthritis. (71,72,91–93) …”

“…(72) They found that it decreases GATA4 expression and improves the ability of osteoprogenitors to differentiate into bone-forming osteoblasts. Moreover, it attenuates the senescence-associated secretory phenotype and the ability of osteoprogenitors to support osteoclast formation.…”

“…Reproduced with permission from Kim and colleagues. (72) ( D ) Western blot analysis of femoral extracts from female C57BL6/J mice. ( E ) mRNA levels by qPCR in extracts from 7-month-old ( n = 9) and 21-month-old ( n = 10) female C57BL6/J mice.…”