The HECTD1-Rubicon Axis: A Novel Pathway in Osteoarthritis Pathogenesis

Osteoarthritis (OA) is a prevalent and debilitating joint disease, impacting millions worldwide. It is not only the most common form of arthritis but also a primary contributor to chronic pain and disability, especially within the elderly population. Despite significant investigations to understand the pathophysiology of OA, the available interventions for this condition remain limited. Current treatment approaches primarily focus on mitigating pain and improving joint function, yet they often do not tackle the underlying disease mechanisms. As a result, there has been a growing interest in the development of disease-modifying therapies aimed at slowing down or halting the progression of OA.  Chondrocytes which are the specialized cells in cartilage, play a central role in maintaining cartilage health and are key players in the development of OA. Consequently, understanding the mechanisms that lead to chondrocyte dysfunction is crucial for developing effective treatments for OA. Ubiquitination, a post-translational modification process that regulates protein turnover, signal transduction, and various cellular functions, has been implicated in numerous diseases. The final step of ubiquitination is mediated by E3 ubiquitin ligases, which determine the specificity of target proteins for ubiquitin tagging. Despite the well-established importance of ubiquitination in cellular processes, the role of E3 ligases in OA remains not understood. Another important cellular process, autophagy, is a conserved catabolic pathway responsible for the degradation and recycling of cellular components. Dysfunctional autophagy has been linked to various diseases, including autoimmune conditions, cancer, and neurodegenerative disorders. Interestingly, emerging evidence suggests that autophagy plays a protective role in OA pathogenesis. Notably, the expression of autophagy-related genes has been found to be reduced in chondrocytes from OA patients, highlighting the potential significance of autophagy modulation in OA treatment. In a new study published in the peer-reviewed Journal Arthritis & Rheumatology, a team of researchers led by Dr. Shiyao Liao, Dr. Qiangqiang Zheng, Dr. Haotian Shen, Dr. Guang Yang, Dr. Yuzi Xu, Dr. Xiaolei Zhang, Dr. Hongwei Ouyang, and Dr. Zongyou Pan from Zhejiang University builds upon previous research indicating that autophagy is impaired in OA chondrocytes. Specifically, the researchers focused on Class III phosphatidylinositol 3-kinase (PI3KC3), which catalyzes the generation of phosphatidylinositol 3-phosphate and is a key regulator of autophagy. They explored the role of Rubicon protein, which inhibits autophagy by suppressing the VPS34-UVRAG complex, and its potential regulation by the E3 ubiquitin ligase, HECTD1.

To investigate potential E3 ubiquitin ligases involved in OA pathogenesis, the authors conducted RNA sequencing analysis on cartilage samples from healthy individuals and OA patients. Their findings revealed that 2 E3 ubiquitin ligase genes were up-regulated, while 26 were down-regulated in OA cartilage samples. Of these, HECTD1 showed the most significant down-regulation, prompting further investigation. Moreover, their Immunohistochemical analysis of human and mouse cartilage samples confirmed that HECTD1 expression decreased with the severity of OA. Additionally, HECTD1 expression was found to decline in an age-associated OA mouse model, suggesting a connection between HECTD1 expression and OA progression. To explore the in vivo role of HECTD1 in OA pathogenesis, the researchers employed both overexpression and knockout models. Overexpression of HECTD1 in mouse knee joint tissues resulted in less severe OA progression, as evidenced by histologic examination and OARSI scores. Conversely, conditional knockout of Hectd1 in chondrocytes exacerbated OA progression and increased chondrocyte death. These findings indicated that HECTD1 plays a protective role in OA development.

The authors confirmed the in vivo relevance of the HECTD1-Rubicon axis in autophagy regulation in mouse models. HECTD1 overexpression in mouse knee joint tissues promoted autophagy, while Hectd1 knockout inhibited autophagy, especially in aging-related OA. This suggests that HECTD1-mediated regulation of autophagy through Rubicon is a key factor in OA pathogenesis. The researchers also explored the potential therapeutic implications of their findings. Rubicon expression was found to be significantly increased in both human and mouse OA cartilage samples. To further investigate the role of Rubicon in OA pathogenesis, the researchers knocked down Rubicon in mice after inducing OA. This resulted in less severe cartilage destruction, lower OARSI scores, and reduced osteophyte formation, highlighting the potential of Rubicon as a therapeutic target for OA. Lastly, the study investigated the impact of Rubicon ubiquitination on autophagy. Ectopic expression of Rubicon-K534R, a mutant incapable of being ubiquitinated at Lys534, further inhibited autophagosome formation compared to wild-type Rubicon. This finding underscores the importance of Rubicon ubiquitination at this specific site in regulating autophagy.

The study led by Dr. Zongyou Pan and colleagues has provided valuable information into the pathogenesis of OA and unveiled a previously unrecognized molecular pathway involving the E3 ubiquitin ligase HECTD1 and the autophagy regulator Rubicon. These findings contribute to our understanding of how autophagy is modulated in OA progression, offering potential therapeutic targets for this debilitating disease. By investigating the role of HECTD1, the researchers have built a compelling case for the protective function of HECTD1 in OA development. Moreover, their identification of the specific ubiquitination site (Lys534) on Rubicon sheds light on the precise molecular mechanisms governing autophagy regulation. The study’s findings have several clinical implications. First, the identification of Rubicon as a potential therapeutic target for OA opens new avenues for drug development. Strategies aimed at reducing Rubicon expression or enhancing its degradation through ubiquitination could potentially alleviate OA progression and its associated symptoms.