Model links proteins from aging knee tissue to gene activity in cartilage cells
Model links proteins from aging knee tissue to gene activity in cartilage cells
On August 22, Aging Cell published a study that traced the path from proteins in the extracellular environment to the cellular response. The authors combined data on age-related changes in knee adipose tissue with a map of gene activity in cartilage, then compared the model’s predictions with results from a previously published cell experiment.
Chondrocytes, the cells that make up cartilage, are located near the infrapatellar fat pad, an adipose tissue structure within the knee joint. The fat pad releases proteins into its surroundings, and the composition of these proteins changes with age.
In a 2025 study, aged mouse chondrocytes responded differently to conditioned media from young and aged fat pads. Medium from young fat pads increased the production of extracellular matrix proteins, which form the material between cells that supports cartilage. This observation led the researchers to ask which receptors, the proteins on the cell surface that receive external signals, allow the surrounding medium to alter gene activity inside a chondrocyte.
In an experiment with brain cells, four interventions reduced markers of aging when signals passed from astrocytes to astrocytes, but had no effect when signals passed from microglia to astrocytes. The source of the signal changed the outcome. In the joint, the path must therefore be traced from an external protein to its receptor and then to the cell’s gene expression program.
The authors selected 30 proteins whose levels differed between media from young and aged fat pads. Using the NicheNet database, they identified receptors capable of recognizing these proteins. They then mapped these receptors onto a network of gene interactions in human cartilage, constructed from 18 healthy samples and 20 samples from people with knee osteoarthritis. The algorithm starts with the receptors and ranks genes according to their proximity to those receptors within the network.
The model was tested against data from a previously published experiment. Aged mouse chondrocytes were cultured for 72 hours in medium from either young or aged fat pads. The researchers then measured which genes had become more or less active. A total of 84 genes differed between the two conditions. The model and the experiment converged on cellular respiration programs, including the assembly of complex I in the mitochondrial respiratory chain. Cells exposed to aged medium also showed weaker staining for SDHA, a protein associated with mitochondrial function.
The model converts differences in the tissue environment into a sequence that can guide the next experiment: an external protein, a receptor on the chondrocyte, and a change in gene activity that needs to be tested. It helps narrow the set of receptors for further experiments.