Inhibiting stimulator of interferongenes (STING) signaling in bone marrow macrophages may attenuate age-related bone loss and improve skeletal outcomes.
The experimental study involved genetically modified and wild-type male mice aged 3 to 24 months. Researchers used myeloid-specific gene deletion, macrophage-targeted gene knockdown, pharmacologic intervention, and cellular and molecular assays to investigate the role of STING signaling during skeletal aging. They also examined interactions among bone marrow adipocytes, macrophages, and mesenchymal stem cells to determine how age-associated changes in the marrow microenvironment influence bone remodeling.
Outcomes such as bone mineral density, trabecular bone volume fraction, trabecular number, thickness, and separation using microcomputed tomography were evaluated. Additional assessments included bone formation, marrow adipocyte accumulation, inflammatory signaling, mitochondrial function, and nicotinamide adenine dinucleotide (NAD)-positive metabolism. These outcomes were examined across different ages and following genetic or pharmacologic modulation of the STING pathway.
The researchers observed progressive activation of STING signaling in bone marrow macrophages with advancing age. Mice with myeloid-specific STING deletion exhibited lower bone mass during early adulthood compared with wild-type controls, whereas the pattern reversed during later aging. From 10 to 24 months, genetically modified mice experienced attenuated bone loss. Although bone mass was higher in these mice at 24 months, the difference at that individual time point was not statistically significant.
To determine whether inhibiting STING beginning in middle age could protect against skeletal deterioration, the researchers administered an adeno-associated virus vector targeting STING in bone marrow macrophages from 12 to 18 months. Treated mice had statistically significantly higher bone mineral density, trabecular bone volume fraction, and trabecular number, along with reduced trabecular separation compared with controls. Histologic analyses also indicated enhanced bone formation and reduced marrow adipocyte accumulation.
The researchers evaluated a combination of the STING inhibitor H-151 and nicotinamide mononucleotide (NMN) supplementation beginning at 12 months. By 18 months, treated mice exhibited higher bone mineral density, trabecular bone volume fraction, and trabecular number, as well as lower trabecular separation compared with controls. The improvements in bone microarchitecture were comparable to those observed with alendronate sodium.
The combination treatment increased mineral apposition rates and reduced expression of inflammatory and senescence-associated markers in bone marrow. In mesenchymal stem cells, the researchers observed improved mitochondrial function and higher NAD-positive levels, suggesting that treatment affected the inflammatory environment and metabolic processes associated with bone formation.
They examined proteins secreted by aging bone marrow adipocytes and identified soluble dipeptidylpeptidase 4 (DPP4) as a contributor to STING activation. Molecular binding experiments demonstrated that soluble DPP4 interacted directly with Toll-like receptor 4 on bone marrow macrophages, cooperating with accumulated mitochondrial DNA to activate STING signaling.
Further analysis showed that STING activation increased expression of CD38 through downstream inflammatory signaling. Elevated CD38 activity amplified inflammation and reduced the availability of NMN. This depletion impaired NAD-positive metabolism in neighboring bone marrow mesenchymal stem cells, contributing to mitochondrial dysfunction and reduced osteogenic differentiation.
The researchers analyzed whether targeting CD38 could reproduce the skeletal benefits observed following STING inhibition. Macrophage-targeted CD38 knockdown from 12 to 18 months attenuated bone loss and improved mitochondrial function and osteogenic differentiation in mesenchymal stem cells. The findings supported the involvement of the STING–CD38 pathway in age-related deterioration of the bone marrow microenvironment.
Although the genetic model targeted bone marrow macrophages, STING deletion also occurred in other myeloid cells, including osteoclast precursors, preventing complete exclusion of their contributions. The approach used to target marrow adipocytes could also affect stromal and adipogenic progenitor cells. Relatively broad observation intervals and the absence of bone mass assessments at 1 to 2 months limited characterization of STING activity during early skeletal development. The combined H-151 and NMN experiment did not include individual treatment groups, preventing determination of the separate contributions of either intervention. The findings were derived from preclinical models and have not established efficacy or safety in humans.
“[T]hese findings position STING signaling in bone marrow macrophages as a pivotal integration hub,” wrote lead study author Zhichao Li, of the Shenzhen Institute of Advanced Technology at the Chinese Academy of Sciences, and colleagues.
The study authors declared no conflicts of interest.
Source: Advanced Science
