Senescence-associated gene pathways are differentially expressed in equine aging-related osteoarthritis.

Overview

• This study investigates how genes associated with cellular aging (senescence) are expressed differently in the joints and blood cells of horses with osteoarthritis (OA), a degenerative joint disease linked to aging.
• The findings suggest that senescence-related gene activity increases in joint cells but decreases in blood immune cells, indicating potential for targeted therapies that address these aging processes to treat OA in horses.

Background and Objective

• Osteoarthritis (OA) is a common joint disease connected with aging, causing joint degeneration and pain.
• Cellular senescence refers to a state where cells stop dividing and release inflammatory factors, potentially contributing to disease progression such as OA.
• The study aimed to understand differences in expression of senescence-associated genes locally in the joint and systemically in blood immune cells of horses with OA.
• Understanding these differences could pave the way for senotherapeutics—drugs targeting senescent cells—to manage or treat OA.

Methods

• Samples analyzed included synovial fluid cells and peripheral blood mononuclear cells (PBMCs) from 65 horses, both with and without OA.
• Two advanced sequencing approaches were used:
  • Single-cell RNA sequencing on synovial fluid cells to examine gene expression at the individual cell level within affected joints.
  • Bulk mRNA sequencing on PBMCs to assess systemic gene expression in blood immune cells.
• The researchers used a custom gene set of 3,043 genes linked to cellular senescence, based on published data, to identify relevant gene expression changes.
• Differential expression analysis was performed using the limma statistical framework to compare OA horses to controls.
• Senescence genes were categorized into 8 distinct groups, and gene set enrichment analyses (using fast gene set enrichment analysis with permutation testing) calculated scores indicating upregulation or downregulation of these senescence pathways.

Key Findings

• Cell type-specific heterogeneity: Analysis of synovial fluid cells showed that the expression of senescence genes varied significantly among different cell types in the joint environment.
• Inflammatory and stress-induced senescence upregulated: Specific immune cell types in the joint—dendritic cells, cycling cells (actively dividing cells), CD8 T cells, and gamma delta T cells—showed increased expression of genes linked to inflammatory and stress-induced senescence.
• Senescence-associated secretory phenotype (SASP) pathways: These pathways, representing the release of inflammatory and tissue remodeling factors by senescent cells, were mainly active in cycling cells within the joint.
• Key upregulated senescence genes in joints: Genes such as aryl hydrocarbon receptor (AHR), IL-1 receptor antagonist (IL1RN), heme oxygenase 1 (HMOX1), plasminogen activator urokinase receptor (PLAUR), and tissue inhibitor of metalloproteinase 1 (TIMP1) were heightened across various cell types in the synovial fluid, suggesting a role in OA pathology.
• Contrasting downregulation in blood immune cells: In PBMCs from OA horses, most senescence-related gene categories were found to be downregulated, indicating a different systemic response.

Interpretation and Implications

• The study reveals a contrasting pattern of senescence gene expression: increased activity in the joint microenvironment versus decreased activity in circulating immune cells.
• This suggests that senescence contributes locally within the joint to OA, potentially exacerbating inflammation and tissue damage, while systemic senescence expression is reduced or altered.
• Targeted therapies that clear senescent cells or modulate their secretory profiles (senotherapeutics) could be promising strategies for modifying OA disease progression in horses.
• Such approaches might reduce joint inflammation and degradation by addressing the local senescence burden, potentially improving outcomes for aging horses with OA.

Summary

• This research provides crucial insights into how aging and cellular senescence contribute to equine OA at both joint and systemic levels.
• The findings support further exploration of senescence-targeting treatments to manage OA, offering hope for developing disease-modifying therapies in veterinary medicine.