Rheumatoid arthritis (RA) is a chronic, systemic autoimmune inflammatory joint disease [1]. The global prevalence of RA is nearly 1 %, and it is more frequently observed in females than males [2]. The current treatment involves the use of methotrexate (MTX) as an anchor drug along with biological disease-modifying anti-rheumatic drugs (bDMARDs) and targeted synthetic DMARDs like Janus kinase (JAK) inhibitors that inhibit multiple cytokine-mediated inflammation [3]. Despite this, durable clinical remission is seen only in a subset of RA patients, and newer targets are warranted to develop new therapeutics [4].

In RA, the synovial joints are the primary target of immune-mediated inflammation, though other organs like the lung and eye can be affected. A synovial joint comprises of two bone surfaces enclosed within a fibrous capsule lined with the synovial membrane. Usually, the synovial membrane is 1–2 thin layer membrane composed of synoviocytes. However, in RA, this membrane undergoes a drastic transformation and becomes multilayered thick [5]. This synovial hyperplasia contributes to joint damage and impacts quality of life of RA patient [[6], [7], [8]]. During the initial phase of the disease progression, several stimuli such as danger-associated molecular patterns (DAMPs), synovial protein citrullination, complement activation, and auto-antibodies activate the fibroblast-like synoviocytes (FLS) cells. This activation leads to the release of inflammatory mediators, matrix metalloproteinases (MMPs), and the recruitment of immune cells, which in turn creates an inflammatory and migratory microenvironment in the synovial space [[9], [10], [11]].

Interestingly, in vivo migration of RA-FLS has been documented which clearly shows the metastasis of the disease from the site of the inflamed joint to the other anatomical locations [12,13]. Moreover, the invasiveness of FLS in vitro is linked to radiologic evidence of joint destruction [14]. Several genes such as Huntingtin-interacting protein 1 (HIP1) [15], transient receptor potential vanilloid subfamily, type 2 channel (TRPV2) [16], and cadherin-11 [17] have been implicated in RA-FLS invasion. The RA-FLS cells abundantly express cadherin-11 at the cartilage and pannus junction, which promotes cell motility and invasion. Consequently, inhibition of cadherin-11 diminishes the invasive capacity of RA-FLS. [18]. Studies have also implicated altered chemokine signaling, MMP pathway, cytokine signaling pathway etc. in RA-FLS invasion [19,20]. Multiple groups have suggested that the invasive nature of FLS is significant in the establishment of RA and targeting altered FLS invasion might be a new therapy in RA [5,21,22]. However, the mechanism underlying the invasive behavior of FLS is not fully understood, highlighting the need to identify new pathways that regulate this process.

Mitochondria are commonly known to regulate Ca2+ homeostasis via mitochondrial calcium uniporter (MCU) [23]. The MCU protein complex localizes in the inner mitochondrial membrane and this alone is essential and adequate for mitochondrial calcium uptake [24]. A number of proteins contribute to the formation and regulation of this complex, including MCU, EMRE, MCUb, MICU1, and MICU2 [25,26]. Studies show the correlation of mitochondrial calcium uptake via MCU [27]with tumor growth and metastatic potential. Even more, silencing of MCU hampered cell motility and invasiveness of the cancer cells [28,29]. However, the role of mitochondrial calcium signaling in RA pathogenesis is unexplored.

In order to maintain their shape, quality, distribution, size, and function, mitochondria undergo orchestrated cycles of fission and fusion processes, a phenomenon termed as mitochondrial dynamics [30,31]. Recent studies indicate that mitochondrial dynamics are essential contributors to diverse cellular functions such as immune response, cell metabolism, migration, cell differentiation, apoptosis, etc. [30,32,33]. Interestingly, mitochondrial fission protein Dynamin-related protein-1(Drp1) regulates the invasiveness of migrating cells [34,35], and the smaller mitochondrial pool in the leading edge helps in migration by producing reactive oxygen species (ROS) [36]. Previous studies have documented that increased MCU expression enhances mitochondrial Ca2+ uptake, which stimulates mitochondrial biogenesis and, in turn, supports colorectal cancer (CRC) cell growth both in vitro and in vivo. Consequently, CRC cells with MCU knockdown showed reduced expression of mitochondrial fission gene, Drp1and elevated levels of mitochondrial fusion gene, OPA1 mitochondrial dynamin-like GTPase, suggesting that MCU-mediated mitochondrial Ca2+ uptake may enhance mitochondrial fission while inhibiting mitochondrial fusion [37].

We hypothesized that mitochondrial MCU-mediated calcium handling might regulate FLS migration and invasion to the surrounding tissues. Using RA patient-derived primary FLS cells and transgenic animal model of RA, here we show that MCU regulates FLS invasion by regulating mitochondrial function and actin dynamics. We also demonstrate the molecular mechanism underlying the cross-talk between mitochondrial calcium shuttling and mitochondrial dynamics in regulating FLS migration during RA pathogenesis.