The SLC22 family includes organic anion transporters (OATs), organic cation transporters (OCTs) and organic carnitine and zwitterion transporters (OCTNs). These are often referred to as drug transporters even though they interact with many endogenous metabolites and signaling molecules (Nigam, S.K., Nature Reviews Drug Discovery, 14:29–44, 2015). Phylogenetic analysis of SLC22 supports the view that these transporters may have evolved over 450 million years ago. Many OAT members were found to appear after a major expansion of the SLC22 family in mammals, suggesting a physiological and/or toxicological role during the mammalian radiation. Putative SLC22 orthologs exist in worms, sea urchins, flies, and ciona. At least six groups of SLC22 exist. OATs and OCTs form two Major clades of SLC22, within which (apart from Oat and Oct subclades), there are also clear Oat-like, Octn, and Oct-related subclades, as well as a distantly related group we term “Oat-related” (which may have different functions). Based on available data, it is arguable whether SLC22A18, which is related to bacterial drug-proton antiporters, should be assigned to SLC22. Disease-causing mutations, single nucleotide polymorphisms (SNPs) and other functionally analyzed mutations in OAT1, OAT3, URAT1, OCT1, OCT2, OCTN1, and OCTN2 map to the first extracellular domain, the large central intracellular domain, and transmembrane domains 9 and 10. These regions are highly conserved within subclades, but not between subclades, and may be necessary for SLC22 transporter function and functional diversification. Our results not only link function to evolutionarily conserved motifs but indicate the need for a revised sub-classification of SLC22.

Specific interactions of growth factors with heparan sulfate are thought to regulate stages of branching morphogenesis in developing mammalian organs, but the evidence derives mostly from studies of explanted tissues or cell culture. We recently provided in vivo evidence that inactivation of Ndst1, the predominant N-deacetylase/N-sulfotransferase gene essential for the formation of mature heparan sulfate, results in a highly specific defect in murine lobuloalveolar development. Here, we demonstrate a highly penetrant dramatic defect in primary branching by mammary epithelial-specific inactivation of Ext1, a subunit of the copolymerase complex that catalyzes the formation of the heparan sulfate chain. In contrast to Ext1 deletion, inactivation of Hs2st (which encodes an enzyme required for 2-O-sulfation of uronic acids in heparan sulfate) did not inhibit ductal formation but displayed markedly decreased secondary and ductal side-branches as well as fewer bifurcated terminal end buds. Targeted conditional deletion of c-Met, the receptor for HGF, in mammary epithelial cells showed similar defects in secondary and ductal side-branching, but did not result in any apparent defect in bifurcation of terminal end buds. Although there is published evidence indicating a role for 2-O sulfation in HGF binding, primary epithelial cells isolated from Hs2st conditional deletions were able to activate Erk in the presence of HGF and there appeared to be only a slight reduction in HGF-mediated c-Met phosphorylation in these cells compared to control. Thus, both c-Met and Hs2st play important, but potentially independent, roles in secondary and ductal side-branching. When considered together with previous studies of Ndst1-deficient glands, the data presented here raise the possibility of partially-independent regulation by heparan sulfate-dependent pathways of primary ductal branching, terminal end bud bifurcation, secondary branching, ductal side-branching and lobuloalveolar formation.

In patients with liver or kidney disease, it is especially important to consider the routes of metabolism and elimination of small-molecule pharmaceuticals. Once in the blood, numerous drugs are taken up by the liver for metabolism and/or biliary elimination, or by the kidney for renal elimination. Many common drugs are organic anions. The major liver uptake transporters for organic anion drugs are organic anion transporter polypeptides (OATP1B1 or SLCO1B1; OATP1B3 or SLCO1B3), whereas in the kidney they are organic anion transporters (OAT1 or SLC22A6; OAT3 or SLC22A8). Since these particular OATPs are overwhelmingly found in the liver but not the kidney, and these OATs are overwhelmingly found in the kidney but not liver, it is possible to use chemoinformatics, machine learning (ML) and deep learning to analyze liver OATP-transported drugs versus kidney OAT-transported drugs. Our analysis of >30 quantitative physicochemical properties of OATP- and OAT-interacting drugs revealed eight properties that in combination, indicate a high propensity for interaction with “liver” transporters versus “kidney” ones based on machine learning (e.g., random forest, k-nearest neighbors) and deep-learning classification algorithms. Liver OATPs preferred drugs with greater hydrophobicity, higher complexity, and more ringed structures whereas kidney OATs preferred more polar drugs with more carboxyl groups. The results provide a strong molecular basis for tissue-specific targeting strategies, understanding drug–drug interactions as well as drug–metabolite interactions, and suggest a strategy for how drugs with comparable efficacy might be chosen in chronic liver or kidney disease (CKD) to minimize toxicity.

Background: Mammary epithelial branching morphogenesis depends on proper sulfation of heparan sulfate proteoglycans. Results: The absence of both Ndst1 and Ndst2 induces increased branching of the ductal epithelium. Conclusion: N-Sulfation regulates primary and secondary branching events in the developing mammary gland. Significance: Stages of ductal branching and lobuloalveolar formation are regulated by distinct sets of heparan sulfate biosynthetic enzymes.

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