Modulation of Urea Transport Attenuates TLR2-Mediated Microglial Activation and Upregulates Microglial Metabolism In Vitro.

Background: Alzheimer's disease (AD) is a neurodegenerative disorder traditionally characterised by the presence of amyloid beta (Aβ) plaques and neurofibrillary tau tangles in the brain. However, emerging research has highlighted additional metabolic hallmarks of AD pathology. These include the metabolic reprogramming of microglia in favour of glycolysis over oxidative phosphorylation. This shift is attributed to an 'M1'-like pro-inflammatory phenotype, which exacerbates neuroinflammation and contributes to neuronal damage. The urea cycle also presents as an altered metabolic pathway in AD, due to elevated urea levels and altered expression of urea cycle enzymes, metabolites, and transporters in the brain. However, to date, these changes remain largely unexplored. Methods: This study focuses on understanding the effects of extracellular urea and urea transporter-B (UT-B) inhibition on inflammatory changes in lipoteichoic acid (LTA)-stimulated BV2 microglia and on the viability of SH-SY5Y neuronal cells under oxidative stress and neurotoxic conditions. Results: In BV2 microglia, UT-B inhibition demonstrated a notable anti-inflammatory effect by reducing the formation of nitric oxide (NO) and the expression of tumour necrosis factor α (TNFα) and CCL2 in response to stimulation with the toll-like receptor (TLR)2 agonist, lipoteichoic acid (LTA). This was accompanied by a reduction in extracellular urea and upregulation of UT-B expression. The application of exogenous urea was also shown to mediate the inflammatory profile of BV2 cells in a similar manner but had only a modest impact on UT-B expression. While exposure to LTA alone did not alter the microglial metabolic profile, inhibition of UT-B upregulated the expression of genes associated with both glycolysis and fatty acid oxidation. Conversely, neither increased extracellular urea nor UT-B inhibition had a significant impact on cell viability or cytotoxicity in SH-SY5Y neurones exposed to oxidative stressors tert-butyl hydroperoxide (t-BHP) and 6-hydroxydopamine (6-OHDA). Conclusions: This study further highlights the involvement of urea transport in regulating the neuroinflammation associated with AD. Moreover, we reveal a novel role for UT-B in maintaining microglial metabolic homeostasis. Taken together, these findings contribute supporting evidence to the regulation of UT-B as a therapeutic target for intervention into neuroinflammatory and neurodegenerative disease.