*Article* **Global Gene Expression Analysis Identifies Age-Related Di**ff**erences in Knee Joint Transcriptome during the Development of Post-Traumatic Osteoarthritis in Mice**

**Aimy Sebastian 1, Deepa K. Murugesh 1, Melanie E. Mendez 1,2, Nicholas R. Hum 1,2, Naiomy D. Rios-Arce 1, Jillian L. McCool 1,2, Blaine A. Christiansen <sup>3</sup> and Gabriela G. Loots 1,2,\***


Received: 6 December 2019; Accepted: 29 December 2019; Published: 6 January 2020

**Abstract:** Aging and injury are two major risk factors for osteoarthritis (OA). Yet, very little is known about how aging and injury interact and contribute to OA pathogenesis. In the present study, we examined age- and injury-related molecular changes in mouse knee joints that could contribute to OA. Using RNA-seq, first we profiled the knee joint transcriptome of 10-week-old, 62-week-old, and 95-week-old mice and found that the expression of several inflammatory-response related genes increased as a result of aging, whereas the expression of several genes involved in cartilage metabolism decreased with age. To determine how aging impacts post-traumatic arthritis (PTOA) development, the right knee joints of 10-week-old and 62-week-old mice were injured using a non-invasive tibial compression injury model and injury-induced structural and molecular changes were assessed. At six-week post-injury, 62-week-old mice displayed significantly more cartilage degeneration and osteophyte formation compared with young mice. Although both age groups elicited similar transcriptional responses to injury, 62-week-old mice had higher activation of inflammatory cytokines than 10-week-old mice, whereas cartilage/bone metabolism genes had higher expression in 10-week-old mice, suggesting that the differential expression of these genes might contribute to the differences in PTOA severity observed between these age groups.

**Keywords:** osteoarthritis; aging; PTOA; gene expression; RNA-seq; cartilage degeneration; scRNA-seq; chondrocytes
