Defensa de Tesis Doctoral – Daniel Cañada García

Chronic pain is a complex and debilitating condition with a significant personal, social and economic impact. In western countries, one of the major causes is osteoarthritis (OA), a degenerative joint disease. Nerve growth factor (NGF) and its receptor, TrkA, are key targets in the study of pain mechanisms, as they play a central role in the development and plasticity of nociceptors. Although NGF blocking antibodies have shown efficacy in reducing OA pain, they cause severe side effects. Therefore, understanding the mechanisms by which the NGF/TrkA axis contributes to osteoarthritic pain remains a critical research priority. Ubiquitination is a primary regulatory mechanism of the NGF/TrkA axis. TrkA undergoes ubiquitination upon NGF activation, which directs its internalisation, cellular trafficking and degradation. Previous research in our laboratory has characterised the P782S mutation of TrkA, which disrupts the binding of the E3 ubiquitin ligase Nedd4l to the receptor. This mutation leads to an overactivated NGF/TrkA axis and hypersensitivity to heat and inflammatory pain, and, in particular, to mechanical stimulation in an experimental model of osteoarthritis. TrkA activation leads to gene expression changes that contribute to chronic OA pain. However, transcriptomic studies in sensory ganglia are scarce and this research is the first to focus on NGF/TrkA-mediated transcriptomic changes using a TrkA gain-of-function model. By inducing knee OA in wild-type and TrkA P782S mutant mice, we explored gene expression changes specifically mediated by TrkA during OA progression. Our primary goal was to identify genes and pathways that are directly regulated by the NGF/TrkA axis in the context of OA that contribute to the regulatory mechanisms of NGF/TrkA signalling in nociception and pain processing. Transcriptomic analysis of L3-L5 dorsal root ganglia (DRG) has revealed numerous genes and pathways previously associated with pain and OA which are now confirmed to be regulated by the NGF/TrkA axis in this context; moreover, DEGs and pathways previously associated with NGF/TrkA signalling that we have found to be altered in OA. These include transcriptional regulators such as TP53 and SMAD2, 3 and 4 heterotrimers, and signalling cascades mediated by receptor tyrosine kinases and the MAPK family. We also highlight new potential targets for OA pain research, such as Aak1, Kalrn and Kidins220, which are known in other pain contexts but remain unexplored in OA. Among these DEGs, we also identified Ndfip2, an adaptor and modulator of the Nedd4 family of E3 ubiquitin ligases, as a gene whose expression is controlled by the NGF/TrkA axis under pain conditions. We have performed further investigation due to its potential regulatory role as a second main goal of this project. Our experiments have confirmed its subcellular localisation in the endoplasmic reticulum and Golgi-derived vesicles also in sensory neurons. We have shown that Ndfip2 selectively interacts with TrkA in dorsal root ganglia neurons, but not with Nedd4l. Using genetic strategies to modulate Ndfip2 expression, we have shown that it regulates TrkA levels within its biosynthetic pathway, influencing receptor glycosylation, membrane levels and activation, rather than NGF-mediated ubiquitination. This effect is more pronounced in male mice. Conditional knockout of Ndfip2 in TrkA-expressing neurons in male and female mice has shown that Ndfip2 is critical for mechanical and cold nociception, but does not affect responses to heat or inflammatory pain. Furthermore, Ndfip2 modulates sex differences in response to nociceptive stimuli, increasing sexual dimorphism in cold and formalin-induced pain, but decreasing it in heat and mechanical nociception. This phenotype may be explained by our findings that Ndfip2 interacts with and regulates TRPM8 and Piezo2 ion channels, which are essential for cold and mechanical sensation respectively. In conclusion, our study has identified several genes and pathways involved in OA pain that are regulated by the NGF/TrkA axis. Among them, Ndfip2 emerges as a key modulator of membrane receptors relevant to nociception. Taken together, these findings pave the way for the understanding of NGF/TrkA signalling regulation, providing valuable insights that may contribute to future pain research.