Fifty years ago, increased whole-blood serotonin levels, or hyperserotonemia, first linked disrupted 5-HT homeostasis to Autism Spectrum Disorders (ASDs). The 5-HT transporter (SERT) gene (SLC6A4) has been associated with whole blood 5-HT levels and ASD susceptibility. Previously, we identified multiple gain-of-function SERT coding variants in children with ASD. Here we establish that transgenic mice expressing the most common of these variants, SERT Ala56, exhibit elevated, p38 MAPK-dependent transporter phosphorylation, enhanced 5-HT clearance rates and hyperserotonemia. These effects are accompanied by altered basal firing of raphe 5-HT neurons, as well as 5HT 1A and 5HT 2A receptor hypersensitivity. Strikingly, SERT Ala56 mice display alterations in social function, communication, and repetitive behavior. Our efforts provide strong support for the hypothesis that altered 5-HT homeostasis can impact risk for ASD traits and provide a model with construct and face validity that can support further analysis of ASD mechanisms and potentially novel treatments. development | monoamine | neurotransmitter A utism spectrum disorder (ASD) is a male-predominant disorder that is characterized by deficits in social interactions and communication, as well as repetitive behavior (1). Hyperserotonemia, or increased whole-blood serotonin [i.e., 5-hydroxytryptamine (5-HT)], is a well replicated biomarker that is present in approximately 30% of subjects with ASD (2, 3). Some data suggest an association of hyperserotonemia with stereotyped or self-injurious behavior, but results have been inconsistent (4, 5). Despite the high heritability of whole-blood 5-HT levels (6), a mechanistic connection between hyperserotonemia and specific components of the pathophysiology of ASD remains enigmatic. In blood, 5-HT is contained almost exclusively in platelets (7) that lack 5-HT biosynthetic capacity but accumulate the monoamine via the antidepressant-sensitive serotonin transporter (SERT; 5-HTT). A genome-wide study of whole-blood 5-HT as a quantitative trait found association with the SERT-encoding gene SLC6A4, as well as with ITGB3, which encodes the SERT-interacting protein integrin β3. In both cases, the strongest evidence for association was found in males (8-10). Linkage studies in ASD also implicate the 17q11.2 region containing SLC6A4, again with stronger evidence in males (11, 12).As common SLC6A4 variants are only modestly associated with ASD (13), we and our colleagues previously screened SLC6A4 for rare variants in multiplex families that demonstrate strong linkage to 17q11.2. In this effort, we identified five rare SERT coding variants, each of which confers increased 5-HT transport in transfected cells as well as in lymphoblasts derived from SERT variant-expressing probands (11,14,15). We found the most common of these variants, Ala56 (allele frequency in subjects of European ancestry of 0.5-1%), to be overtransmitted to autism probands, and to be associated with both rigid-compulsive behavior and sensory aversion (11,16). N...

The catecholamine dopamine (DA) functions as a powerful modulatory neurotransmitter in both invertebrates and vertebrates. As in man, DA neurons in the nematode Caenorhabditis elegans express a cocaine-sensitive transporter (DAT-1), presumably to regulate synaptic DA signaling and limit DA spillover to extrasynaptic sites, although evidence supporting this is currently lacking. In this report, we describe and validate a novel and readily quantifiable phenotype, swimming-induced paralysis (SWIP) that emerges in DAT-1-deficient nematodes when animals exert maximal physical activity in water. We verify the dependence of SWIP on DA biosynthesis, vesicular packaging, synaptic release, and on the DA receptor DOP-3. Using DAT-1 specific antibodies and GFP::DAT-1 fusions, we demonstrate a synaptic enrichment of DAT-1 that is achieved independently of synaptic targeting of the vesicular monoamine transporter (VMAT). Importantly, dat-1 deletions and point mutations that disrupt DA uptake in cultured C. elegans neurons and/or impact DAT-1 synaptic localization in vivo generate SWIP. SWIP assays, along with in vivo imaging of wild-type and mutant GFP::DAT-1 fusions identify a distal COOH terminal segment of the transporter as essential for efficient somatic export, synaptic localization and in vivo DA clearance. Our studies provide the first description of behavioral perturbations arising from altered trafficking of DATs in vivo in any organism and support a model whereby endogenous DA actions in C. elegans are tightly regulated by synaptic DAT-1.

The neurotransmitter dopamine (DA) modulates brain circuits involved in attention, reward, and motor activity. Synaptic DA homeostasis is primarily controlled via two presynaptic regulatory mechanisms, DA D 2 receptor (D 2 R)-mediated inhibition of DA synthesis and release, and DA transporter (DAT)-mediated DA clearance. D 2 Rs can physically associate with DAT and regulate DAT function, linking DA release and reuptake to a common mechanism. We have established that the attention-deficit hyperactivity disorder-associated human DAT coding variant Ala559Val (hDAT A559V) results in anomalous DA efflux (ADE) similar to that caused by amphetamine-like psychostimulants. Here, we show that tonic activation of D 2 R provides support for hDAT A559V-mediated ADE. We determine in hDAT A559V a pertussis toxin-sensitive, CaMKII-dependent phosphorylation mechanism that supports D 2 R-driven DA efflux. These studies identify a signaling network downstream of D 2 R activation, normally constraining DA action at synapses, that may be altered by DAT mutation to impact risk for DA-related disorders.

The authors note that, due to a printer's error, on page 3787, right column, first paragraph, lines 8-20, "Although significant evidence supports a primary role for the 5-HT transporter in the reinforcing properties of psychostimulants (35-37), SERT blockade also appears to contribute to reinforcement (38-40).Indeed, SERT appears to be primarily responsible for the sustained reinforcing properties of cocaine in the 5-HT transporter-KO mouse (22,23,(39)(40)(41). The significant compensatory alterations evident in 5-HT transporter-KO mice encouraged Chen and colleagues (42, 43) to develop a mouse bearing knock-in mutations in 5-HT transporter that, in vitro, reduced cocaine potency. Studies with these mice have yielded convincing evidence that 5-HT transporter is a key determinant of many synaptic and behavioral actions of cocaine" should instead appear as "Although significant evidence supports a primary role for the DA transporter in the reinforcing properties of psychostimulants (35-37), SERT blockade also appears to contribute to reinforcement (38-40). Indeed, SERT appears to be primarily responsible for the sustained reinforcing properties of cocaine in the DA transporter-KO mouse (22,23,(39)(40)(41). The significant compensatory alterations evident in DA transporter-KO mice encouraged Chen and colleagues (42,43) to develop a mouse bearing knock-in mutations in the DA transporter that, in vitro, reduced cocaine potency. Studies with these mice have yielded convincing evidence that the DA transporter is a key determinant of many synaptic and behavioral actions of cocaine." www.pnas.org/cgi

The presynaptic dopamine (DA) transporter (DAT) is a major determinant of synaptic DA inactivation, an important target for psychostimulants including cocaine and amphetamine, and a mediator of DA neuron vulnerability to the neurotoxins 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridinium ion. To exploit genetic approaches for the study of DATs and neural degeneration, we exploited the visibility of green fluorescent protein (GFP)-tagged DA neurons in transgenic nematodes to implement a forward genetic screen for suppressors of 6-OHDA sensitivity. In our initial effort, we identified three novel dat-1 alleles conferring 6-OHDA resistance. Two of the dat-1 alleles derive from point mutations in conserved glycine residues (G55, G90) in contiguous DAT-1 transmembrane domains (TM1 and TM2, respectively), whereas the third allele results in altered translation of the transporter's COOH terminus. Our studies reveal biosynthetic, trafficking and functional defects in the DAT-1 mutants, exhibited both in vitro and in vivo. These studies validate a forward genetic approach to the isolation of DA neuronspecific toxin suppressors and point to critical contributions of the mutated residues, as well as elements of the DAT-1 COOH terminus, to functional expression of catecholamine transporters in neurons.