# LOXL2 deletion triggers TMJ osteoarthritis, while overexpression protects it from NF-κB-induced chondrocyte apoptosis

## TL;DR

Deleting the LOXL2 gene worsens TMJ osteoarthritis, while increasing it protects cartilage by reducing inflammation and cell death.

## Contribution

This study reveals LOXL2 as a novel protective factor in TMJ-OA through its anti-inflammatory and anti-apoptotic mechanisms.

## Key findings

- LOXL2 deletion increases inflammatory and cartilage-degrading gene expression in TMJ cartilage.
- LOXL2 treatment reduces IL-1β-induced chondrocyte apoptosis and protects mitochondrial function.
- LOXL2 mitigates NF-κB signaling and ECM degradation in TMJ-OA.

## Abstract

Temporomandibular joint osteoarthritis (TMJ-OA) affects a significant proportion of the population worldwide. However, there has been no substantial progress in the development of FDA-approved drugs for treatment due to a lack of understanding of the specific factors regulating key TMJ-OA molecular mechanisms. Lysyl Oxidase-Like-2 (LOXL2) promotes knee joint cartilage protection and is downregulated in a TMJ-OA animal model. We evaluated the role of LOXL2 in TMJ cartilage, its molecular mechanism, and gene networks using in vivo Loxl2 knockout mice (Acan-Cre; Loxl2flox/flox) and ex vivo goat TMJ cartilage. Our results show that Loxl2 knockout in mouse cartilage upregulates Il1b, Mmp9, Mmp13, Adamts4, and Adamts5, but reduces the levels of aggrecan and proteoglycan. Loxl2 deleted TMJ cartilage show a higher enrichment of inflammatory response, TNFA signaling via NF-κB, extracellular matrix (ECM), and collagen degradation pathway network. Conversely, LOXL2 treatment reduces interleukin-1 beta (IL-1β)-induced expression of Mmp13, protects mitochondrial function, and ECM from degeneration. Importantly, LOXL2 attenuates IL-1β-induced chondrocyte apoptosis via the phosphorylation of NF-κB and expression of the pain-related gene PTGS2 (encodes COX2). Taken together, Loxl2 knockout mice exacerbate TMJ-OA through cartilage/ECM degradation, mitochondrial dysfunction, chondrocyte apoptosis, and inflammatory gene expression, whereas LOXL2 treatment mitigate these effects.