Photoaffinity Labeling and Quantitative Chemical Proteomics Identify Liver X Receptor β as the Functional Target of Enhancers of Astrocytic Apolipoprotein E

Apolipoprotein E (apoE) is a 34 kDa key lipid transport protein in both the brain and the periphery. In the brain, it is produced predominantly by astrocytes and has been shown to play an important role in promoting neurite outgrowth and synaptogenesis, clearing amyloid β42, promoting cerebrovascular integrity and in neuroimmune modulation. Given the critical role of apoE in the brain, it is important to understand its regulation in the CNS. To that end, utilizing a phenotypic screen, we identified novel chemical matter that increased astrocytic apoE secretion in vitro. We designed a clickable photoaffinity probe and carried out quantitative chemical proteomics to identify liver x receptor β (LXRβ) as the target. Binding of the ligand stabilized LXRβ, as shown by CETSA. Our findings demonstrated that the lead chemical matter bound directly to LXRβ, and highlight the power of chemical proteomics to identify the target of a phenotypic screening hit.