The SLC6 transporters: perspectives on structure, functions, regulation, and models for transporter dysfunction

Abstract

The human SLC6 family is composed of approximately 20 structurally related symporters (co-transporters) that use the transmembrane electrochemical gradient to actively import their substrates into cells. Approximately half of the substrates of these transporters are amino acids, with others transporting biogenic amines and/or closely related compounds, such as nutrients and compatible osmolytes. In this short review, five leaders in the field discuss a number of currently important research themes that involve SLC6 transporters, highlighting the integrative role they play across a wide spectrum of different functions. The first essay, by Gary Rudnick, describes the molecular mechanism of their coupled transport which is being progressively better understood based on new crystal structures, functional studies, and modeling. Next, the question of multiple levels of transporter regulation is discussed by Reinhard Krämer, in the context of osmoregulation and stress response by the related bacterial betaine transporter BetP. The role of selected members of the human SLC6 family that function as nutrient amino acid transporters is then reviewed by François Verrey. He discusses how some of these transporters mediate the active uptake of (essential) amino acids into epithelial cells of the gut and the kidney tubule to support systemic amino acid requirements, whereas others are expressed in specific cells to support their specialized metabolism and/or growth. The most extensively studied members of the human SLC6 family are neurotransmitter reuptake transporters, many of which are important drug targets for the treatment of neuropsychiatric disorders. Randy Blakely discusses the role of posttranscriptional modifications of these proteins in regulating transporter subcellular localization and activity state. Finally, Dennis Murphy reviews how natural gene variants and mouse genetic models display consistent behavioral alterations that relate to altered extracellular neurotransmitter levels.

Fig. 1

Alternating access mechanisms and LeuT. a Similar conformational changes could account for symport (left) and antiport (right) using different rules. Prohibited conformational changes are indicated by a red bar. b The reaction cycle of LeuT is initiated by binding of two Na⁺ ions (upper left) followed by substrate binding (upper right). Conformational changes to the occluded state and the inward-open state follow (right). Dissociation of Na⁺ and substrate leads to the apo-state (lower left) which can re-orient to the outward open form (left). c–e Binding sites for Na1 and substrate (c) and Na2 in outward-open (d) and inward-open (e) structures. Na1 binding site residues and transmembrane helices are numbered

Fig. 2

Fine-tuning of BetP-mediated betaine uptake. Upon a hyperosmotic upshift (A > B) which leads to an increase in the cytoplasmic K⁺ concentration, BetP switches into the active state and actively accumulates betaine in the cell. When osmotic adaptation is reached, BetP activity ceases (B >C). Upon application of a second upshift to higher external osmolality (C>D), the cycle of activation and adaptation (C>D, D >E) is initiated again

Fig. 3

Cellular uptake of essential amino acids (EAAs). Only a small number of transporters mediate the uptake of essential amino acids. Uniporters, as shown in panel a, equilibrate the concentration of aromatic or branched chain EAAs across the plasma membrane. Panel b shows a heterodimeric antiporter (obligatory exchanger) that can exchange intracellular amino acids (e.g., non-essential amino acids (NEAAs) taken up by other transporters) against extracellular EAAs (tertiary-active transport). Panels c and d (epithelial cell) show the only two symporters known to actively import EAAs into cells using the driving force of ion gradients (secondary-active transport)

Fig. 4

Model for SERT-cytoskeletal interactions dictating cell surface transporter regulation. In the resting state, SERT is present in two compartments, one that permits free diffusion in the membrane (left), and a second compartment that represents confinement to membrane microdomains (center) where transporters are immobilized by cytoskeleton-associated proteins (middle). When cytoskeleton- associated constraints are relaxed in response to PKG/IL-1β/p38 MAPK activation (or through actin destabilizers or C-SERT peptide treatments), SERT remains confined to membrane microdomains (right), though now transporters can adopt conformations that favor increased transport activity. Question mark overlying transitions into and out of membrane microdomains denotes the possibility that such movements could also play a role in SERT regulation, though they are not features of the PKG and p38 MAPK-dependent SERT regulation detected in the current study. Adapted from Chang et al. [15]