Scientists at The Scripps Research Institute (TSRI) report why the risk of osteoarthritis (OA) increases as we age. They also describe a new approach for developing new therapies to maintain healthy joints.

The study (“FoxO Transcription Factors Modulate Autophagy and Proteoglycan 4 in Cartilage Homeostasis and Osteoarthritis”), which appears in Science Translational Medicine, suggests that FOXO proteins are responsible for the maintenance of healthy cells in the cartilage of our joints.

“FoxO transcription factors protect against cellular and organismal aging, and FoxO expression in cartilage is reduced with aging and in OA. To investigate the role of FoxO in cartilage, Col2Cre-FoxO1, 3, and 4 single knockout (KO) and triple KO mice (Col2Cre-TKO) were analyzed. Articular cartilage in Col2Cre-TKO and Col2Cre-FoxO1 KO mice was thicker than in control mice at 1 or 2 months of age. This was associated with increased proliferation of chondrocytes of Col2Cre-TKO mice in vivo and in vitro. OA-like changes developed in cartilage, synovium, and subchondral bone between 4 and 6 months of age in Col2Cre-TKO and Col2Cre-FoxO1 KO mice.

“Col2Cre-FoxO3 and FoxO4 KO mice showed no cartilage abnormalities until 18 months of age when Col2Cre-FoxO3 KO mice had more severe OA than control mice. Autophagy and antioxidant defense genes were reduced in Col2Cre-TKO mice. Deletion of FoxO1/3/4 in mature mice using Aggrecan(Acan)-CreERT2 (AcanCreERT-TKO) also led to spontaneous cartilage degradation and increased OA severity in a surgical model or treadmill running. 

“The superficial zone of knee articular cartilage of Col2Cre-TKO and AcanCreERT-TKO mice exhibited reduced cell density and markedly decreased Prg4. In vitro, ectopic FoxO1 expression increased Prg4 and synergized with transforming growth factor-β stimulation. In OA chondrocytes, overexpression of FoxO1 reduced inflammatory mediators and cartilage-degrading enzymes, increased protective genes, and antagonized interleukin-1β effects. 

“Our observations suggest that FoxO [transcription factors] play a key role in postnatal cartilage development, maturation, and homeostasis and protect against OA-associated cartilage damage.”

“We discovered that FoxO transcription factors control the expression of genes that are essential for maintaining joint health,” says Martin Lotz, M.D., a TSRI professor and senior author of the study. “Drugs that boost the expression and activity of FoxO could be a strategy for preventing and treating OA.”

For the study, researchers used mouse models with FoxO deficiency in cartilage to see how the FoxO proteins affect maintenance of cartilage throughout adulthood. The researchers noticed a striking difference in the mice with knockout FoxO deficiency. Their cartilage degenerated at much younger age than in control mice. The FoxO-deficient mice also had more severe forms of posttraumatic OA induced by meniscus damage, and these mice were more vulnerable to cartilage damage during treadmill running. 

Turns out the FoxO-deficient mice had defects in autophagy and in mechanisms that protect cells from damage by oxidants. Specific to cartilage, FoxO-deficient mice did not produce enough lubricin, a lubricating protein that normally protects the cartilage from friction and wear. This lack of lubricin was associated with a loss of healthy cells in a cartilage layer of the knee joint called the superficial zone.

These problems all came down to how FoxO proteins work as transcription factors to regulate gene expression. Without FoxO proteins acting as regulators, expression of inflammation-related genes increases, causing pain, while levels of autophagy-related genes plummet, leaving cells without a way to repair themselves.

“The housekeeping mechanisms, which keep cells healthy, were not working in these knockout mice,” Dr. Lotz explains. 

To determine whether targeting FoxO has therapeutic benefits, the investigators used genetic approaches to increase FoxO expression in cells of humans with OA and found that the levels of lubricin and protective genes returned to normal.

The next step in this research is to develop molecules that enhance FoxO and test them in experimental models of OA.

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