Slc10A4 - what do we know about the function of this "secret ligand carrier" protein?

SLC10A4 is a vesicular amine-associated transporter modulating dopamine homeostasis

BACKGROUND

The neuromodulatory transmitters, biogenic amines, have profound effects on multiple neurons and are essential for normal behavior and mental health. Here we report that the orphan transporter SLC10A4, which in the brain is exclusively expressed in presynaptic vesicles of monoaminergic and cholinergic neurons, has a regulatory role in dopamine homeostasis.

METHODS

We used a combination of molecular and behavioral analyses, pharmacology, and in vivo amperometry to assess the role of SLC10A4 in dopamine-regulated behaviors.

RESULTS

We show that SLC10A4 is localized on the same synaptic vesicles as either vesicular acetylcholine transporter or vesicular monoamine transporter 2. We did not find evidence for direct transport of dopamine by SLC10A4; however, synaptic vesicle preparations lacking SLC10A4 showed decreased dopamine vesicular uptake efficiency. Furthermore, we observed an increased acidification in synaptic vesicles isolated from mice overexpressing SLC10A4. Loss of SLC10A4 in mice resulted in reduced striatal serotonin, noradrenaline, and dopamine concentrations and a significantly higher dopamine turnover ratio. Absence of SLC10A4 led to slower dopamine clearance rates in vivo, which resulted in accumulation of extracellular dopamine. Finally, whereas SLC10A4 null mutant mice were slightly hypoactive, they displayed hypersensitivity to administration of amphetamine and tranylcypromine.

CONCLUSIONS

Our results demonstrate that SLC10A4 is a vesicular monoaminergic and cholinergic associated transporter that is important for dopamine homeostasis and neuromodulation in vivo. The discovery of SLC10A4 and its role in dopaminergic signaling reveals a novel mechanism for neuromodulation and represents an unexplored target for the treatment of neurological and mental disorders.

Quantitative trait loci mapping and gene network analysis implicate protocadherin-15 as a determinant of brain serotonin transporter expression

Presynaptic serotonin (5-hydroxytryptamine, 5-HT) transporters (SERT) regulate 5-HT signaling via antidepressant-sensitive clearance of released neurotransmitter. Polymorphisms in the human SERT gene (SLC6A4) have been linked to risk for multiple neuropsychiatric disorders, including depression, obsessive-compulsive disorder and autism. Using BXD recombinant inbred mice, a genetic reference population that can support the discovery of novel determinants of complex traits, merging collective trait assessments with bioinformatics approaches, we examine phenotypic and molecular networks associated with SERT gene and protein expression. Correlational analyses revealed a network of genes that significantly associated with SERT mRNA levels. We quantified SERT protein expression levels and identified region- and gender-specific quantitative trait loci (QTLs), one of which associated with male midbrain SERT protein expression, centered on the protocadherin-15 gene (Pcdh15), overlapped with a QTL for midbrain 5-HT levels. Pcdh15 was also the only QTL-associated gene whose midbrain mRNA expression significantly associated with both SERT protein and 5-HT traits, suggesting an unrecognized role of the cell adhesion protein in the development or function of 5-HT neurons. To test this hypothesis, we assessed SERT protein and 5-HT traits in the Pcdh15 functional null line (Pcdh15(av-) (3J) ), studies that revealed a strong, negative influence of Pcdh15 on these phenotypes. Together, our findings illustrate the power of multidimensional profiling of recombinant inbred lines in the analysis of molecular networks that support synaptic signaling, and that, as in the case of Pcdh15, can reveal novel relationships that may underlie risk for mental illness.