Evolution of substrate specificity for the bile salt transporter ASBT (SLC10A2)

Abstract

The apical Na+-dependent bile salt transporter (ASBT/SLC10A2) is essential for maintaining the enterohepatic circulation of bile salts. It is not known when Slc10a2 evolved as a bile salt transporter or how it adapted to substantial changes in bile salt structure during evolution. We characterized ASBT orthologs from two primitive vertebrates, the lamprey that utilizes early 5α-bile alcohols and the skate that utilizes structurally different 5β-bile alcohols, and compared substrate specificity with ASBT from humans who utilize modern 5β-bile acids. Everted gut sacs of skate but not the more primitive lamprey transported 3H-taurocholic acid (TCA), a modern 5β-bile acid. However, molecular cloning identified ASBT orthologs from both species. Cell-based assays using recombinant ASBT/Asbt's indicate that lamprey Asbt has high affinity for 5α-bile alcohols, low affinity for 5β-bile alcohols, and lacks affinity for TCA, whereas skate Asbt showed high affinity for 5α- and 5β-bile alcohols but low affinity for TCA. In contrast, human ASBT demonstrated high affinity for all three bile salt types. These findings suggest that ASBT evolved from the earliest vertebrates by gaining affinity for modern bile salts while retaining affinity for older bile salts. Also, our results indicate that the bile salt enterohepatic circulation is conserved throughout vertebrate evolution.

Origin and evolution of transporter substrate specificity within the NPF family

“…Further subfunctionalization within the GTR clade led to the evolution of the GTR3 subclade identified as transporters with preference and high affinity for indole glucosinolate. Thus, the subfunctionalization within the GTRs from broad to narrow specificity is contrary to the evolutionary dynamics proposed previously for substrate-transport evolution, where progenitor transporters had a narrow substrate specificity that expanded during evolution to become increasingly broad (Lionarons et al, 2012).

10.7554/eLife.19466.025Figure 7.Model of the evolution of the glucosinolate NPF transporter specificity.We propose that diversification of an ancestral high-affinity cyanogenic glucoside transporter (exemplified by MeCGTR1) lead to a dual-specificity transporter capable of transporting both cyanogenic glucosides and glucosinolates (exemplified by Me14G074000).

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Sodium-dependent bile salt transporters of the SLC10A transporter family: more than solute transporters

“…Traditionally, bile acids and bile salts were identified as sole ASBT substrates (34,41). Lately, evidence has been presented supporting the concept that bile salt transport capacity of ASBT is phylogenetically conserved (101). Interestingly, a bacterial homologue of ASBT is also capable of mediating bile salt transport (78).…”

A novel class of apical sodium--dependent bile salt transporter inhibitors: 1-(2,4-bifluorophenyl)-7-dialkylamino-1,8-naphthyridine-3-carboxamides

“…A human ASBT expression construct was prepared as previously described 22 . The inhibitory activity was expressed as inhibition (%) in 10 µmol/L (Table 1).…”