International Journal of Molecular Sciences Review Molecular Regulation of Canalicular ABC Transporters Amel Ben Saad 1,2,† , Alix Bruneau 2,3,† , Elodie Mareux 1,† , Martine Lapalus 1 , Jean-Louis Delaunay 2, Emmanuel Gonzales 1,4 , Emmanuel Jacquemin 1,4, Tounsia Aït-Slimane 2,‡ and Thomas Falguières 1,*,‡ 1 Physiopathogénèse et Traitement des Maladies du foie, Inserm, Université Paris-Saclay, UMR_S 1193, Hepatinov, 91400 Orsay, France; [email protected] (A.B.S.); [email protected] (E.M.); [email protected] (M.L.); [email protected] (E.G.); [email protected] (E.J.) 2 Centre de Recherche Saint-Antoine (CRSA), Inserm, Sorbonne Université, UMR_S 938, Institute of Cardiometabolism and Nutrition (ICAN), 75012 Paris, France; [email protected] (A.B.); [email protected] (J.-L.D.); [email protected] (T.A.-S.) 3 Department of Hepatology and Gastroenterology, Charité Universitäts Medizin Berlin, 13353 Berlin, Germany 4 Paediatric Hepatology and Paediatric Liver Transplant Department, Reference Center for Rare Paediatric Liver Diseases, FILFOIE, ERN RARE LIVER, Assistance Publique-Hôpitaux de Paris, Faculté de Médecine Paris-Saclay, CHU Bicêtre, 94270 Le Kremlin-Bicêtre, France * Correspondence: [email protected]; Tel.: +33-(0)1-69-15-62-94 † These authors share first co-authorship. ‡ These authors share last co-authorship. Abstract: The ATP-binding cassette (ABC) transporters expressed at the canalicular membrane of hepatocytes mediate the secretion of several compounds into the bile canaliculi and therefore play a key role in bile secretion. Among these transporters, ABCB11 secretes bile acids, ABCB4 translocates phosphatidylcholine and ABCG5/G8 is responsible for cholesterol secretion, while ABCB1 and ABCC2 transport a variety of drugs and other compounds. The dysfunction of these transporters Citation: Ben Saad, A.; Bruneau, A.; Mareux, E.; Lapalus, M.; Delaunay, leads to severe, rare, evolutionary biliary diseases. The development of new therapies for patients J.-L.; Gonzales, E.; Jacquemin, E.; with these diseases requires a deep understanding of the biology of these transporters. In this review, Aït-Slimane, T.; Falguières, T. we report the current knowledge regarding the regulation of canalicular ABC transporters’ folding, Molecular Regulation of Canalicular trafficking, membrane stability and function, and we highlight the role of molecular partners in these ABC Transporters. Int. J. Mol. Sci. regulating mechanisms. 2021, 22, 2113. https://doi.org/ 10.3390/ijms22042113 Keywords: bile secretion; ABCB1; ABCB4; ABCB11; ABCC2; ABCG5/G8; molecular partners Academic Editor: Jose J. G. Marin Received: 1 February 2021 1. Introduction Accepted: 18 February 2021 Published: 20 February 2021 One of the liver’s main functions is bile production and secretion. In addition to its digestive function, bile plays an important role in detoxification. Bile secretion is me- Publisher’s Note: MDPI stays neutral diated by several ATP-binding cassette (ABC) transporters, which are expressed at the with regard to jurisdictional claims in canalicular membrane of hepatocytes. The main canalicular ABC transporters are the bile published maps and institutional affil- salt export pump (BSEP, ABCB11), which transports bile acids (BAs), ABCB4 also known iations. as multidrug resistance protein 3 (MDR3) translocating phosphatidylcholine (PC) and ABCG5/G8 excreting cholesterol [1]. BA, PC and cholesterol form mixed micelles in the aqueous environment of bile. In addition to these compounds, bile contains a wide variety of drugs and organic anions, which are secreted by ABCB1, also known as multidrug resis- tance protein 1 (MDR-1, or P-glycoprotein) and ABCC2 (multidrug resistance-associated Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. protein 2, MRP2), respectively [2,3]. ABC transporters share a common basic architecture This article is an open access article and similar ATP-driven functions. They are organized in two repeats, each containing distributed under the terms and a membrane-spanning domain (MSD) with six transmembrane (TM) helices and a cyto- conditions of the Creative Commons plasmic nucleotide-binding domain (NBD), those two moieties being connected by an Attribution (CC BY) license (https:// intracellular linker. The MSDs ensure substrate recognition and translocation, whereas creativecommons.org/licenses/by/ NBDs, which are highly conserved among all ABC transporters, provide the energy for 4.0/). this process [3]. In contrast to other canalicular ABC transporters, ABCG5 and ABCG8 Int. J. Mol. Sci. 2021, 22, 2113. https://doi.org/10.3390/ijms22042113 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 2113 2 of 17 whereas NBDs, which are highly conserved among all ABC transporters, provide the Int. J. Mol.energy Sci. 2021 for, 22, 2113this process [3]. In contrast to other canalicular ABC transporters, ABCG5 and 2 of 17 ABCG8 are half transporters that require heterodimerization to ensure their function, and ABCC2 has a third MSD at its N-terminus [4]. The key role of canalicular ABC transporters in bile secretion is highlightedare half transporters by their that implication require heterodimerization in a wide range to ensure of theirdiseases function, such and as ABCC2 cholestasis (ABCB4,has ABCB11), a third MSD sitosterolem at its N-terminusia (ABCG5/G8), [4]. The key role Dubin–Johnson of canalicular ABC transporterssyndrome in bile (ABCC2) and cancersecretion (ABCB1) is highlighted (Figure 1) by [3]. their To implication develop in new a wide therapies range of diseasesfor patients such as with cholestasis diseases related to deficient(ABCB4, ABCB11),canalicular sitosterolemia ABC transporters, (ABCG5/G8), it is Dubin–Johnson crucial to better syndrome understand (ABCC2) and cancer (ABCB1) (Figure1)[ 3]. To develop new therapies for patients with diseases related the molecular mechanismsto deficient regulating canalicular ABC the transporters,traffic and it isfunction crucial to betterof these understand transporters. the molecular Several studies havemechanisms reported regulating that the the bios trafficynthesis, and function trafficking of these transporters. and activity Several of studiesABC have transporters are regulatedreported thatby thenumerous biosynthesis, molecular trafficking partners and activity, most of ABC of transporters which have are regulatedbeen by identified by two-hybridnumerous screens molecular using partners, liver most banks of which [3]. have Targeting been identified these by two-hybridinteractors screens using liver banks [3]. Targeting these interactors represents a potential therapeutic option represents a potentialfor therapeu patients. Thistic reviewoption focuses for patients. on molecular This regulators review offocuses canalicular on molecular ABC transporters regulators of canalicularinvolved ABC in biletransporters formation. involved in bile formation. FigureFigure 1. ATP-binding1. ATP-binding cassette cassette (ABC) (ABC) transporters transporters at the canalicular at the canalicular membrane of membrane hepatocytes. of The hepatocytes. bile canaliculus is formedThe bile by thecanaliculus canalicular is membrane formed by of the hepatocytes. canalicula Ther membrane main canalicular of hepatocytes. ABC transporters The aremain indicated canalicular according toABC the naturetransporters of their substrates:are indicated in yellow according for hydrophobic to the nature substrates of their and substrates: in blue for in drugs. yellow Note for that ATP8B1 is nothydrophobic an ABC transporter substrates but aand P-type in blue ATPase. for drugs. However, Note its functionthat ATP8B1 is tightly is not related an ABC to the transporter other canalicular but a ABC transporters.P-type ATPase. The substrates However, of these its function transporters is tightly are shown related in black, to andthe theirother flows canalicular are indicated ABC by transporters. black arrows. The mainThe diseasessubstrates associated of these with transpor functionalters defects are shown of these in transporters black, and are their indicated flows in are red. indicated PFIC: progressive by black familial intrahepaticarrows. The cholestasis. main diseases associated with functional defects of these transporters are indicated in red. PFIC: progressive2. familial Folding intrahepatic and Glycosylation cholestasis. of Canalicular ABC Transporters Protein folding is a highly regulated process that is mediated by numerous factors, 2. Folding and Glycosylationincluding folding of Canalicular proteins and molecularABC Transporters chaperones [5 ]. Some of these proteins have been Protein foldingshown is a highly as interactors regulated of canalicular process ABC that transporters, is mediated controlling by numerous their biosynthesis factors, and folding. As most transmembrane proteins, ABC transporter biosynthesis starts with their including folding proteinscotranslational and translocationmolecular chap and insertionerones into[5]. the Some endoplasmic of these reticulum proteins (ER) have membrane been shown as interactorsthrough of the canalicular Sec61 translocon ABC complex transporters, [6]. Numerous controlling accessory their factors biosynthesis were described to and folding. As mostfacilitate transmembrane the translocation proteins process,, ABC including transporter the translocating biosynthesis chain-associated starts with membrane their