Higher SLC7A2 expression is associated with abnormal neuroinflammatory response and nitrosative stress in Huntington's disease
Background: We previously identified solute carrier family 7 member 2 (SLC7A2) as one of the top up-regulated genes when normal huntingtin was deleted. SLC7A2 has a high affinity for Arginine. Arginine is implicated in inflammatory responses and SLC7A2 is an important regulator of innate and adaptive immunity in macrophagy. Although neuroinflammation is clearly demonstrated in HD animal models and patients, the question of whether neuroinflammation actively participates in HD pathogenesis is a topic of ongoing research and debate. Here we study the role of SLC7A2 in mediating neuroinflammatory stress response in HD cells. Methods: RNA sequencing, quantitative RT-PCR and datamining of publicly available RNA-seq datasets of human patients were performed to assess the levels of SLC7A2 mRNA in different HD cellular models and patients. Biochemical studies were then conducted on cell lines and primary mouse astrocytes to investigate Arginine metabolism and nitrosative stress in response to neuroinflammation. The CRISPR-Cas9 system was used to knockout SLC7A2 in STHdhQ7 and Q111 cells to investigate its role in mediating the neuroinflammatory response. Live-cell imaging was used to measure mitochondrial dynamics. Finally, exploratory studies were performed using Enroll-HD periodic human patient datasets to analyze the effect of Arginine supplement on HD progression. Results: We found that SLC7A2 is selectively up-regulated in HD cellular models and patients. HD cells exhibit an overactive response to neuroinflammatory challenges including abnormally high iNOS induction and NO production, leading to increased protein nitrosylation. Depleting extracellular Arg or knocking out SLC7A2 can block the iNOS induction and NO production in STHdhQ111 cells. We further examined the functional impact of protein nitrosylation on a well-documented protein target, DRP-1 and found that more mitochondria were fragmented in challenged STHdhQ111 cells. Lastly, analysis of Enroll-HD datasets suggested that HD patients taking Arginine supplement progressed more rapidly than others. Conclusions: Our data suggest a novel pathway that links Arginine uptake to nitrosative stress via up-regulation of SLC7A2 in the pathogenesis and progression of HD. It further implicates that Arginine supplement may potentially pose a greater risk to HD patients. Overall design: To investigate the global transcriptomal changes in STHdhQ7/Q7 and STHdhQ111/!111 cells in response to neuroinflammation. Neuroinflammation was induced with a combination of 100ng/ml IFNg and 5ug/ml LPS.