The main goals of my research are focused on enhancing our understanding of how drugs of abuse and antidepressants interact with and modulate the function of the dopamine (DAT) and serotonin transporter (SERT) and to identify the cellular processes that are involved in regulating DAT and SERT function.
DAT and SERT are the primary mechanism for clearance of their respective neurotransmitter from the extracellular space. They are targets of psychostimulants, such as cocaine and amphetamines, and of antidepressants such as SSRIs. Additionally, amphetamine-like molecules are in clinical use for the treatment of attention deficit hyperactivity disorder (ADHD). These drugs either block transport of substrates as is the case for SSRIs and cocaine, or are actual substrates, such as amphetamines, which both competitively inhibit uptake and stimulate the release (efflux) of intracellular neurotransmitter. The result is an increase in levels of extracellular dopamine or serotonin augmenting neuronal receptor signaling that ultimately drives the behaviors associated with the respective drugs.
To better understand the cellular processes that regulate transporter function we have used functional expression cloning to identify a MAP kinase phosphatase, MKP3, as a modulator of dopamine transporter (DAT) surface expression. Surprisingly, conventional MAP kinase families are not directly involved in the regulated internalization of DAT. My lab is actively pursuing this paradoxical result to further understand by what mechanism MKP3 modulates transporter function. We hypothesize that this project will reveal novel mechanisms of transporter regulation and speculate that novel therapeutic targets of drug abuse and also affective disorders can be revealed.
In projects aimed at enhancing our understanding of the interaction between DAT and SERT and amphetamine-like molecules we have discovered that the human DAT and SERT appear more promiscuous in substrate selectivity compared to similar transporters from other organisms, including a newly isolated SERT from the human parasite Schistosoma mansoni. In the parasite SERT, unlike in its mammalian counterparts, only serotonin but not other substrates including amphetamine and ecstasy (MDMA) induce efflux. We are using the parasite SERT to identify structural elements within mammalian SERTs and DATs that determine transporter substrate specificity and we will ultimately pursue the specific role of DAT and SERT in amphetamine and MDMA addiction and neurotoxicity by developing knock-in mice with amphetamine and MDMA insensitive serotonin and/or dopamine transporters. Additionally, because the efflux process is the main target of amphetamine-like drugs of abuse, including amphetamine, methamphetamine, and ecstasy, this opens up the intriguing possibility that this process can be therapeutically targeted without interfering with the uptake process - further emphasizing the need for a better understanding of the structural features of the transporter that is involved in these processes.
Selected publications:
"Two allelic isoforms of the serotonin transporter from Schistosoma mansoni display electrogenic transport and high selectivity for serotonin"
Fontana AC, Sonders MS, Pereira-Junior OS, Knight M, Javitch JA, Rodrigues V, Amara SG, Mortensen OV
Eur J Pharmacol, 616(1-3):48-57.(2009)
"Genetic Complementation Screen Identifies a Map Kinase Phosphatase, MKP3, as a Regulator of Dopamine Transporter Trafficking"
Mortensen OV; Larsen MB; Prasad BM; and Amara SG
Mol Biol Cell 19(7):2818-29 (2008)
"Gain of function mutants reveal sites important for the interaction of the atypical inhibitors benztropine and bupropion with monoamine transporters"
Mortensen, OV; and Amara, SG
J Neurochem 98 (5):1531-40 (2006).
"Dynamic regulation of the dopamine transporter"
Mortensen OV; and Amara SG
Eur J Pharmacol 479 (1-3): 159-170 (2003)
"Species scanning mutagenesis of the serotonin transporter reveals residues essential in high-affinity recognition of antidepressants"
Mortensen OV; and Wiborg O
J Neurochem 79 (2):237-47 (2001)
"Functional analysis of a novel human serotonin transporter gene promoter"
Mortensen, OV; Thomassen, M; Larsen, MB; Whittemore, SR and Wiborg O
Brain Res Mol Brain Res 68: 141-148 (1999)
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