Extracellular Matrix Stiffening Contributes to Cartilage Aging and Osteoarthritis

Age-related changes in the structure of the extracellular matrix that surrounds and supports cells are not as well studied as changes in cell behavior. Nonetheless, there is plenty of evidence for changes in the extracellular matrix to negatively affect tissue function. Cells create and maintain the matrix, but the state of the matrix in turn influences cells, and over time is affected by more than just cell behavior. Metabolic processes can alter and fragment elastin, cross-link collagen molecules, and so forth.

Cross-linking of matrix molecules occurs with age as a byproduct of the normal operation of metabolism, reducing flexibility and increasing stiffness. Targeting this cross-linking is a field still in its infancy, and only a few lines of research and development have made significant progress. Clinical trials of cross-link breaking in the lens of the eye have been undertaken, but this chemistry isn't relevant to the rest of the body. Some inroads have been made on finding ways to break down the persistent glucosepane cross-links that appear to be the most relevant to human extracellular matrix aging elsewhere in the body, but despite the launch of a company, Revel Pharmaceuticals, to develop these candidate treatments, there is still a long road ahead.

New mechanism uncovered behind osteoarthritis could inform new treatments

Osteoarthritis occurs when cartilage in a joint stiffens and begins to break down which then damages the underlying bone, resulting in pain, swelling and feelings of stiffness. There are currently no treatments to reverse this cartilage stiffening and resulting damage. Much has remained unknown about the molecular causes of this damage and how to treat it. These unknowns are especially germane to knee osteoarthritis, where no single event causes the cartilage damage, and the greatest predictive risk factor is aging.

Using advanced mass spectrometry technology, the researchers mapped out the trajectory of structural and protein changes in mice with knee osteoarthritis over the course of their lifetimes and according to sex. They then compared their findings to the current understanding of knee osteoarthritis in humans. The researchers found that Klotho was heavily involved in the molecular process that led to osteoarthritis. This work was an extension of previous studies showing that Klotho protects mitochondria within skeletal muscle and plays a key role in skeletal muscle regeneration following injury. As people age, their klotho levels go down, hence why it's referred to as a longevity protein.

The new analysis revealed that when knee cartilage tissue became stiffer, the gene that codes for Klotho was repressed. They verified this in models of young and old chondrocyte cells responsible for cartilage formation, which were seeded in environments designed to mimic young and old tissue stiffness. Young chondrocyte cells looked old when put on a stiff surface due to the loss of Klotho, but when the researchers protected the cells from the stiffness in their environment, they observed chondrocyte health.

Age-related matrix stiffening epigenetically regulates α-Klotho expression and compromises chondrocyte integrity

Extracellular matrix stiffening is a quintessential feature of cartilage aging, a leading cause of knee osteoarthritis. Yet, the downstream molecular and cellular consequences of age-related biophysical alterations are poorly understood. Here, we show that epigenetic regulation of α-Klotho represents a novel mechanosensitive mechanism by which the aged extracellular matrix influences chondrocyte physiology. Using mass spectrometry proteomics followed by a series of genetic and pharmacological manipulations, we discovered that increased matrix stiffness drove Klotho promoter methylation, downregulated Klotho gene expression, and accelerated chondrocyte senescence in vitro.

In contrast, exposing aged chondrocytes to a soft matrix restored a more youthful phenotype in vitro and enhanced cartilage integrity in vivo. Our findings demonstrate that age-related alterations in extracellular matrix biophysical properties initiate pathogenic mechanotransductive signaling that promotes Klotho promoter methylation and compromises cellular health. These findings are likely to have broad implications even beyond cartilage for the field of aging research.