SERT structure and function
Intracellular and extracellular serotonin levels are controlled through tissue SERT expression levels and transporter activity. SERT is a protein consisting of 630 amino acids and has a similar structure to the noradrenaline transporter (NET) and dopamine transporter (DAT). In vitro experiments with SERT have demonstrated that phosphorylation state of the transporters is controlled by several kinase and phosphatase signalling pathways which alters movement of serotonin through the transporter [76]. SERT-medicated serotonin uptake is driven by a Na+/Cl- transmembrane ion gradient [77]. SERT proteins can be regulated by numerous protein kinase (PK) linked pathways, which include the signalling molecules protein kinase C (PKC), protein kinase G (PKG) and p38 mitogen-activated protein kinase (MAPK) [78-80]. Extracellular serotonin can induce the phosphorylation and downregulation of SERT through PKC signalling pathways [81]. Several G-protein coupled receptors such as adenosine, histamine and α2-adrenergic receptors also modulate SERT activity [79,82] as well as inflammatory cytokines such as IL-10 and IFN-γ, and TNF-α [83-85]. SERT is the major mechanism by which serotonin uptake from extracellular fluid occurs; however, when SERT function or expression is altered and the levels of serotonin are elevated, other monoamine transporters that have a lower affinity for serotonin such as DAT and NET will transport serotonin [86].
Gain-of-function SERT-coding variants have been reported with some SNPs in the gene that encodes SERT, SLC6A4, causing a change in the amino acid sequence of SERT [87]. These are rare genetic variants, which represent a frequency of much less that 1% of the population. Human variants such as Ile425Leu, Phe465Leu and Leu550Val when expressed in HeLa cells have been shown to exhibit a gain of serotonin transport phenotype due to elevated expression of the transporter and altered regulation via the PKG/p38 MAPK signalling pathways [88] whereas the Gly56Ala variant increases in serotonin transport across cell membranes with no changes in transporter numbers [89].
The Gly56Ala variant has been reported to have a higher prevalence in individuals diagnosed with ASD and is associated with both sensory aversion and rigid-compulsive behaviour [90], whereas the Ile425Val variant (albeit the same locus, different amino acid substitution to Ile425Leu) is associated with obsessive-compulsive disorder and Aspergers syndrome [91].
To further investigate the gain of function activity of the Gly56Ala variant in vivo, transgenic mice expressing the SERT Ala56 were developed and exhibited normal growth patterns and fertility [92]. Further studies on the transgenic Gly56Ala mice showed they had increased CNS serotonin clearance, enhanced serotonin receptor sensitivity and hyperserotonemia. The mice also exhibited alterations in social function, communication and repetitive behaviours [93]. Similarly in other transgenic mice, overexpression of the SERT protein caused a reduction in the brain region levels of serotonin and enhanced sensitivity of postsynaptic 5-HT2A receptors was observed [94].
In summary, SERT function is regulated via several signalling systems. When SERT tissue expression increases or SERT function is enhanced, serotonin uptake is increased, which diminishes levels of the neurotransmitter in the synaptic cleft and causes an increase in sensitivity of postsynaptic serotonin receptors to serotonin. In human and animal studies where increased SERT function was observed neurological symptoms similar to those observed in ASD were reported.