Immune signal drives bone loss by filling marrow with fat in mice
Bones don’t weaken with age by accident. Inside bone marrow, stem cells that normally build new bone start producing fat cells instead. A new mouse study identifies the immune signal driving this shift, and shows what happens when it’s blocked.
Bone is constantly renewed through a tightly regulated process. Osteoclasts break down old bone, while osteoblasts build new tissue. Osteoblasts arise from bone marrow stem cells. With age, that balance tips: marrow fills with fat, bone-forming activity declines, and fracture risk rises.
An immune pathway misfiring with age
Chinese researchers focused on macrophages, immune cells resident in bone marrow. Specifically, they studied cGAS-STING, a signaling pathway that normally detects foreign DNA from bacteria or viruses. With age, this pathway becomes overactive, responding to leaked mitochondrial DNA as if it were an infection, triggering persistent low-grade inflammation.
The researchers compared bone marrow from mice aged 3, 10, 18, and 24 months. STING pathway activation rose progressively with age, with more cytoplasmic DNA, more signaling molecules, and stronger activation of downstream proteins. Macrophages showed the highest baseline expression of the Sting gene, and their activation increased most with age.
The team then silenced STING specifically in macrophages in mice aged 12 to 18 months. Bone density and structure were significantly better preserved. Marrow fat cell numbers fell, and new bone deposition increased. Macrophages without STING were also less harmful to young stem cells that needed to replenish bone marrow.
Marrow fat cells as upstream triggers
The researchers also identified a possible upstream trigger. Secretions from old marrow fat cells activated macrophage STING. Older mice additionally had higher levels of bacterial waste products in their blood and marrow, which may compound STING overactivation. The precise causal sequence still requires further study.
The study was published in Advanced Science. Results are preliminary and limited to mice; whether the same pathway operates identically in humans has not been established. From a longevity-science perspective, it is notable that a specific immune signaling node appears to govern bone preservation during aging.
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