Elsevier

Journal of Psychiatric Research

Volume 68, September 2015, Pages 261-269
Journal of Psychiatric Research

The amino acid transporter SLC6A15 is a regulator of hippocampal neurochemistry and behavior

https://doi.org/10.1016/j.jpsychires.2015.07.012Get rights and content

Highlights

  • SLC6A15 regulates glutamate content in the hippocampus.

  • Absence of SLC6A15 reduced amino acid levels in the hippocampus.

  • Volume of the hippocampus is not changed following SLC6A15 manipulation.

  • Extracellular glutamate content is regulated by SLC6A15 expression levels in the hippocampus.

Abstract

Although mental disorders as major depression are highly prevalent worldwide their underlying causes remain elusive. Despite the high heritability of depression and a clear genetic contribution to the disease, the identification of genetic risk factors for depression has been very difficult. The first published candidate to reach genome-wide significance in depression was SLC6A15, a neuronal amino acid transporter. With a reported 1,42 fold increased risk of suffering from depression associated with a single nucleotide polymorphism (SNP) in a regulatory region of SLC6A15, the polymorphism was also found to affect hippocampal morphology, integrity, and hippocampus-dependent memory. However, the function of SLC6A15 in the brain is so far largely unknown. To address this question, we investigated if alterations in SLC6A15 expression, either using a full knockout or a targeted hippocampal overexpression, affect hippocampal neurochemistry and consequently behavior. We could show that a lack of SLC6A15 reduced hippocampal tissue levels of proline and other neutral amino acids. In parallel, we observed a decreased overall availability of tissue glutamate and glutamine, while at the same time the basal tone of extracellular glutamate in the hippocampus was increased. By contrast, SLC6A15 overexpression increased glutamate/glutamine tissue concentrations. These neurochemical alterations could be linked to behavioral abnormalities in sensorimotor gating, a key translational endophenotype relevant for many psychiatric disorders. Overall, our data supports SLC6A15 as a crucial factor controlling amino acid content in the hippocampus, thereby likely interfering with glutamatergic transmission and behavior. These findings emphasize SLC6A15 as pivotal risk factor for vulnerability to psychiatric diseases.

Graphical abstract

We here propose the involvement of the neutral amino acid transporter SLC6A15, which has been implicated as risk factor for psychopathology in humans, in regulating the activity of the brain glutamatergic system through the proline-glutamate biosynthesis pathway.

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Introduction

Psychiatric diseases affect millions of people worldwide and are regarded as the number one burden in western societies (Lopez et al., 2006, Gustavsson et al., 2011), but their molecular underpinnings are often still unclear. Recently, the novel candidate gene SLC6A15 was associated with psychiatric disorders as depression (Kohli et al., 2011), stress system activity as well as memory and attention (Schuhmacher et al., 2013). SLC6A15 belongs to the solute carrier family 6 (SLC6A) and encodes for a sodium-dependent transporter for neutral amino acids, mainly localized in neurons. The chemical and structural features of SLC6A15 have been well characterized (Bröer et al., 2006), but its physiological function is still poorly understood. The study from Kohli and colleagues also reported an effect of the risk polymorphism on glutamate levels and hippocampal volume, suggesting a potential link of SLC6A15 and glutamatergic signaling (Kohli et al., 2011). In mice, deletion of SLC6A15 did not result in an obviously different behavioral endophenotype compared to wild-type littermates under basal condition, except for the partial alteration in anxiety-like behavior after acute stress (Drgonova et al., 2007). So far, no experimental proof of a direct connection between the SLC6A15 transporter and glutamate signaling was published.

Among different neurochemical hypotheses explaining mood disorder vulnerability, the glutamate hypothesis has recently gained more attention. For example, the importance of the glutamatergic system in the hippocampus in depression vulnerability has been extensively described (Sanacora et al., 2012). However, an altered glutamate signaling or an imbalance of excitatory and inhibitory neurotransmitters has been proposed as central mechanism for a number of psychiatric disorders, including also schizophrenia (Moghaddam and Javitt, 2012), autism (Tebartz van Elst et al., 2014) or bipolar disorders (Chen et al., 2010). Interestingly, proline, one of the main amino acids transported by SLC6A15, is involved in glutamate synthesis (Pérez-Arellano et al., 2010). This is of specific interest as a number of studies have already correlated proline levels in the brain with differences in sensorimotor gating and memory (Cherkin et al., 1976, Gogos et al., 1999, Roussos et al., 2009), phenomena determined by glutamatergic neurotransmission (Geyer et al., 2001, Riedel et al., 2003) and highly relevant for psychiatric disorders as, e.g., schizophrenia (Braff, 1990). Sensorimotor gating refers to the state-dependent regulation of transmission of forwarding the sensory information to the motor system (Nusbaum and Contreras, 2004). Sensorimotor gating tests are a powerful tool used both in clinical and preclinical studies to assess integrity of the neural circuits.

To test whether SLC6A15 is modulating brain neurochemistry, in particular of the glutamatergic system, and, consequently, behavior, we here studied the effects of either SLC6A15 complete deletion or hippocampus-specific SLC6A15 overexpression in mice. We report the differences in hippocampal amino acid content, including proline, alteration of glutamate and glutamine levels, and impairments of the sensorimotor gating behavior after manipulation of SLC6A15, thereby providing the first direct experimental support for the involvement of SLC6A15 in glutamate-related pathophysiology.

Section snippets

Animals and husbandry

SLC6A15 Knockout mice were kindly provided (Drgonova et al., 2007) and kept as in house colony. SLC6A15KO mice, on a C57Bl/6 background, (from now on abbreviated as SLC-KO) did not show overt reproductive failures and no abnormalities in the survival rate. Mice used for the experiments were obtained from heterozygous breeding pairs and only male animals were used in this study. SLC6A15 wild-type littermates (abbreviated as SLC-WT) were used as a control group. Genotypes were verified by PCR

Experiment 1: Amino acid concentrations in the hippocampus

The total levels of the main amino acids which are primarily substrates for SLC6A15 were measured in hippocampal extracts using mass spectroscopy. SLC-KO mice showed significantly reduced levels of proline, leucine, isoleucine and methionine compared to SLC-WT (proline: Upro = 20, p < 0. 05; leucine: Uleu = 23, p < 0. 05; isoleucine: Uile = 20, p < 0. 05; methionine: Umet = 20, p < 0. 05) (Fig. 1A–D). On the other hand, SLC6A15 overexpression did not alter hippocampal levels of those amino

Discussion

Recent studies have reported a role for SLC6A15 in psychiatric disorders (Kohli et al., 2011) and suggested a potential link to the glutamatergic system, but direct experimental evidence for this association was so far missing. We therefore investigated the role of this novel, highly promising candidate gene in regulation of brain neurochemistry and behavior. We show that modulation of SLC6A15 expression affects the glutamatergic system and a related behavioral phenotype, providing evidence

Contributors

Study design and planning: SS and MVS.

Performing the experiments: SS and CN, EA, TG, JR, BB, SK, KVW, CL, JD.

Data analysis: SS and MC, MU, MVS.

Manuscript preparation: SS and MVS.

All authors have reviewed and approved the final article.

Role of funding source

The Max Planck Institute of Psychiatry funded this study. The funding source had no involvement in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication.

Funding and disclosure

The authors declare no conflict of interest.

Acknowledgments

We warmly thank D. Harbich and B. Schmid for technical help and N. Matosin for proof reading.

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