Review
SLC6 transporters: Structure, function, regulation, disease association and therapeutics

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Abstract

The SLC6 family of secondary active transporters are integral membrane solute carrier proteins characterized by the Na+-dependent translocation of small amino acid or amino acid-like substrates. SLC6 transporters, which include the serotonin, dopamine, norepinephrine, GABA, taurine, creatine, as well as amino acid transporters, are associated with a number of human diseases and disorders making this family a critical target for therapeutic development. In addition, several members of this family are directly involved in the action of drugs of abuse such as cocaine, amphetamines, and ecstasy. Recent advances providing structural insight into this family have vastly accelerated our ability to study these proteins and their involvement in complex biological processes.

Introduction

The solute carrier 6 (SLC6) transport family are secondary active co-transporters that utilize a chemiosmotic Na+ gradient to couple ‘downhill’ transport of Na+ with ‘uphill’ transport of their substrates across a biomembrane. Some members of the SLC6 family co-transport Cl and accordingly were originally named Na+/Cl--dependent transporters. Based on the substrate they translocate, SLC6 transporters can be divided in 4 subgroups. The neurotransmitter transporters (NTT) which include 3 γ-aminobutyric acid (GABA) transporters (GAT), 2 glycine transporters (GLY) and the monoamine (dopamine (DAT), serotonin (SERT) and norepinephrine (NET) transporters; the amino acid transporters which include proline (PROT, IMINO), cationic and neutral amino acid transporters (AA0, AA0,+); the osmolyte transporters which include the betaine (BGT1), taurine (TauT) transporters and creatine transporters (CT) and 1 orphan transporter. The functional divisions of the SLC6 family closely match the groupings observed in phylogenetic analysis (Fig 1). Currently, the SLC6 family is comprised of >300 prokaryotic and eukaryotic proteins of which 21 have been identified in humans (Beuming et al., 2006). It is worth noting that within the “Transporter Classification System” developed by Milton Saier’s group (Saier et al., 2009), the SLC6 transporters are members of the Neurotransmitter: Sodium Symporter (NSS; 2.A.22) family within the amino acid–polyamine–organocation (APC) superfamily. In eukaryotes, the SLC6 carriers that have been characterized fulfill important, if not critical, roles in amino acid transport, (Bröer and Palacín, 2011) homeostasis of osmolytes levels, modulation of neurotransmitter signaling in the central and peripheral nervous system (Kristensen et al., 2011) and spermatogenesis (Chatterjee et al., 2011). Within the intestinal and kidney tubule epithelium these transporters are key in the absorption and reabsorption, respectively, of amino acids and osmolytes that are essential for several physiological processes. Within the nervous system SLC6 transporters are critical in the termination of synaptic transmission for several amino acid and amino acid–derived neurotransmitters in addition to their role in providing essential nutrients and osmolytes to neurons and glial cells (Fig. 2).

In humans, the SLC6 family of transporters defines one of the most clinically relevant protein groups with links to orthostatic intolerance, attention deficit hyperactivity disorder (ADHD) (Mazei-Robison et al., 2008), addiction, osmotic imbalance, X-linked mental retardation (Martínez-Muñoz et al., 2008), Hartnup disorder, hyperekplexia, Tourette syndrome, schizophrenia, Parkinson disease (PD), autism (Hahn and Blakely, 2007) and mood disorders such as depression, anxiety, obsessive compulsive disorder (OCD), and post-traumatic stress disorder (PTSD) (Hahn and Blakely, 2007).

This review will focus on the structure–function aspects of the mammalian SLC6 transporters, their regulation by both classical as well as emerging epigenetic/transgenerational mechanisms and what impact these properties may have on disease and the use of biomarkers to detect these proteins in disease states. For a more comprehensive view of the SLC6 family of proteins see the recent review by (Kristensen et al., 2011).

Section snippets

Structure

The identification of the high-resolution structure of the SLC6 bacterial leucine transporter, LeuT (Yamashita et al., 2005), along with a wealth of supportive biochemical studies (Kristensen et al., 2011) has provided a framework for interpreting SLC6 structure–function relationships. In general, SLC6 proteins have 12 membrane spanning domains (TM) with intracellular N and C termini. In eukaryotic members, the N and C termini are significantly longer and have been shown to mediate complex

Function

Consistent with their diverse physiological role, the SLC6 transporters are found in many tissues including nervous system, kidney, intestine, pancreas, adrenal gland and testis. However, because of their physiological relevance most functional studies have been performed on the NTT subgroup. In the central and peripheral nervous system, the NTTs can regulate signaling among neurons and are a site of action for various drugs.

Regulation

Transporter activity is dependent on the presence of the carrier at the cell surface and the transport capacity of each carrier molecule in this cell surface environment. Although transporter activity can be influenced greatly by changes in transcriptional activity, splice site usage, and mRNA translation and stability, these aspects of SLC6 transporter regulation will not be discussed here but briefly in a later section (see Section 5.1 and (Hahn and Blakely, 2007). Additionally, cellular

Therapeutic development

In this section, we provide information on studies related to epigenetics, mouse models, biomarkers, single nucleotide polymorphisms (SNPs) and small molecule development for their utilization in discovery of novel therapeutics for neurological disorders and drug addiction therapy.

Conclusion and future perspectives

Indeed, the emergence of crystal structures revealing the architecture of a prokaryotic SLC6 transporter complexed with substrates and inhibitors has resulted in an enormous leap forward in our understanding as well as provided a canvas on which to paint our biochemical and genetic studies. Incredibly, we found that this canvas is more universal than first thought and provides insights into unrelated transporters that possess the same LeuT fold core structural design. Nevertheless, the current

Acknowledgments

The authors are grateful for support from the National Institutes of Health to L.K.H. (DA027845), L.C. (DA024797) and J.D.F. (DA031991) and the University North Dakota seed grant program to J.D.F.

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    Publication in part sponsored by the Swiss National Science Foundation through the National Center of Competence in Research (NCCR) TransCure, University of Bern, Switzerland; Director Matthias A. Hediger; Web: http://www.transcure.ch.

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