ApoE is a important protein in lipid metabolism, one of those responsible for transporting cholesterols and other lipids around the body. In today's open access research, the authors present evidence for rising levels of ApoE with aging to degrade the ability of bone to regenerate. This is unfortunate, because it will not be straightforward to just reduce ApoE levels. The protein is vital; a number of serious inherited conditions involve ApoE mutation that leads to greatly increased lipid levels on the bloodstream and organs.
Bone regeneration, and normal tissue maintenance of bone for that matter, is a balance between constant creation and destruction of extracellular matrix structures. Osteoblast cells build bone, and osteoclasts tear it down. Age-related loss of bone density and strength is the result of a growing imbalance that favors osteoclast activity. There is good evidence for numerous mechanisms to be important here, including the usual suspects such as the inflammatory signaling produced by senescent cells. The data here for reversal of loss of regenerative capacity via reduced ApoE levels is quite compelling as an argument for this to be an important proximate mechanism, however.
Researchers confirmed that older people have more Apolipoprotein E, ApoE for short, than younger people. (If that protein name rings a bell, it's because ApoE is also implicated in Alzheimer's and heart disease). The team found that 75-85 year olds had twice as much ApoE in their bloodstreams as 35-45 year olds, then found the same was true for 24-month-old mice versus 4-month-old mice, which approximate the same human age ranges. Next, they wanted to figure out if and how ApoE affects the multi-step process of bone healing. When you break a bone, your body sends signals through the bloodstream to recruit cells to fix it. Some of those recruits, specifically skeletal stem cells, build up cartilage as a temporary scaffolding to hold the fracture together.
In the next step, more recruited cells mature into osteoblasts, bone-building cells, which lay strong, dense bone cells on top of the cartilage scaffolding. Finally, a different kind of cell eats up the cartilage scaffolds and osteoblasts fill those holes with bone. That's if the bone healing process works perfectly. But the researchers found that if they added ApoE to a petri dish with skeletal stem cells, fewer cells developed into osteoblasts and the osteoblasts were worse at building bones. Next, the researchers created an intervention by injecting a virus which keeps mice from making ApoE protein. Circulating ApoE levels dropped by 75 percent and the healed bones contained one and a half-times more strong, hard bone tissue than bones of untreated mice.
In our previous work investigating aged bone regeneration, we identified apolipoprotein E (ApoE) to be one of many candidates potentially involved in aged bone fracture healing. ApoE is a widely expressed lipoprotein classically associated with lipid metabolism and fatty acid transport. ApoE polymorphisms are present in 20% of the population and are associated with hypercholesterolemia, atherosclerosis, and Alzheimer's disease. More recently, clinical evidence has revealed that these ApoE polymorphisms are also associated with decreased bone mineral density and increased risk of hip and vertebral fracture. Mouse models lacking ApoE expression display increased cortical thickness, trabecular number, and bone mineral density. However, a role for ApoE in fracture healing and musculoskeletal aging remains to be investigated.
Here, we sought to understand the role of ApoE in age-associated deficiencies in bone fracture healing. Our previous work has established the importance of circulating factors in the age-associated impairment of bone regeneration. Here, we use our established tibial fracture model coupled with μCT and histological analysis as well as our parabiosis models to identify a role for circulating ApoE in bone fracture healing. We identify ApoE as a negative regulator of osteoblast differentiation and combine this work with functional metabolic assessment and transcript analysis to identify the mechanism by which ApoE influences osteoblast differentiation. Finally, we identify that lowering circulating ApoE levels, using siRNA strategies, in aged mouse models leads to improved bone fracture healing. Collectively, our findings demonstrate that ApoE impairs bone fracture healing in an age-dependent manner by decreasing osteoblast differentiation.