Molecular Insight into the Structure, Function, and Regulation of Bile Acid Transport

Abstract

The human Apical Sodium-dependent Bile Acid Transporter (SLC10A2), also known as hASBT, plays an integral role in the enterohepatic circulation of bile acid and cholesterol homeostasis. As a member of the solute carrier family of membrane transporters, it uses the established Na+ electrochemical gradient as an energy source to reclaim bile salts from the ileum. hASBT has been identified as a promising target for the management of hypercholesterolemia, cholestatic pruritis, and as a prodrug-targeting approach for improved bioavailability of drugs. Limiting the development of such therapeutics, is an incomplete understanding of hASBT's structure. Extensive biochemical and mutagenesis studies for hASBT support a seven transmembrane model. Yet conflicting structures have emerged with the elucidation of the crystal structures of two putative homologues from Neisseria meningitidis (nmAsbt) and Yersinia frederiksenii (yfAsbt). In the absence of a physiological context, the use of their structure as models of the human transporter is limited. In addition to the discrepancy in hASBT's fold, there is limited information regarding the specific proteoforms that are relevant to the functional expression of hASBT and in its regulation. In this work we provide novel molecular insight into the structure, function, and regulation of human ASBT. We contrasted the biochemical, inhibitory, and evolutionary attributes of nmAsbt, yfAsbt, and hASBT and identified their critical differences. The fundamental differences in ion dependency, substrate specificity, and evolutionary context imply divergent structure-function relationships and negate the use of the bacterial transporters as suitable models for hASBT. Additionally, we characterized the role of tyrosine phosphorylation in regulating the functional expression and stability of hASBT. We identified Src family kinases as critical modulators and provide support for hASBT's regulation by phosphatases. As the clinical relevance of PTMs is growing, so too are the number of FDA-approved therapeutics that target these modifications. In this regard, we have made critical advances and gained valuable insight into hASBT's regulation. Finally, we have optimized the biological sample preparation methods and have significantly increased the purity of hASBT samples. When coupled with mass spectrometry analysis, these methods will identify critical proteoforms of hASBT and facilitate a global understanding of its structure-function relationship.