Who Knew? Dopamine Transporter Activity Is Critical in Innate and Adaptive Immune Responses.

The dopamine transporter (DAT) regulates the dimension and duration of dopamine transmission. DAT expression, its trafficking, protein-protein interactions, and its activity are conventionally studied in the CNS and within the context of neurological diseases such as Parkinson's Diseases and neuropsychiatric diseases such as drug addiction, attention deficit hyperactivity and autism. However, DAT is also expressed at the plasma membrane of peripheral immune cells such as monocytes, macrophages, T-cells, and B-cells. DAT activity via an autocrine/paracrine signaling loop regulates macrophage responses to immune stimulation. In a recent study, we identified an immunosuppressive function for DAT, where blockade of DAT activity enhanced LPS-mediated production of IL-6, TNF-α, and mitochondrial superoxide levels, demonstrating that DAT activity regulates macrophage immune responses. In the current study, we tested the hypothesis that in the DAT knockout mice, innate and adaptive immunity are perturbed. We found that genetic deletion of DAT (DAT-/-) results in an exaggerated baseline inflammatory phenotype in peripheral circulating myeloid cells. In peritoneal macrophages obtained from DAT-/- mice, we identified increased MHC-II expression and exaggerated phagocytic response to LPS-induced immune stimulation, suppressed T-cell populations at baseline and following systemic endotoxemia and exaggerated memory B cell expansion. In DAT-/- mice, norepinephrine and dopamine levels are increased in spleen and thymus, but not in circulating serum. These findings in conjunction with spleen hypoplasia, increased splenic myeloid cells, and elevated MHC-II expression, in DAT-/- mice further support a critical role for DAT activity in peripheral immunity. While the current study is only focused on identifying the role of DAT in peripheral immunity, our data point to a much broader implication of DAT activity than previously thought. This study is dedicated to the memory of Dr. Marc Caron who has left an indelible mark in the dopamine transporter field.

Structure-Functional Selectivity Relationship Studies on A-86929 Analogs and Small Aryl Fragments toward the Discovery of Biased Dopamine D1 Receptor Agonists.

Dopamine regulates normal functions such as movement, reinforcement learning, and cognition, and its dysfunction has been implicated in multiple psychiatric and neurological disorders. Dopamine acts through D1- (D1R and D5R) and D2-class (D2R, D3R, and D4R) receptors and activates both G protein- and ß-arrestin-dependent signaling pathways. Current dopamine receptor-based therapies are used to ameliorate motor deficits in Parkinson's disease or as antipsychotic medications for schizophrenia. These drugs show efficacy for ameliorating only some symptoms caused by dopamine dysfunction and are plagued by debilitating side effects. Studies in primates and rodents have shown that shifting the balance of dopamine receptor signaling toward the arrestin pathway can be beneficial for inducing normal movement, while reducing motor side effects such as dyskinesias, and can be efficacious at enhancing cognitive function compared to balanced agonists. Several structure-activity relationship (SAR) studies have embarked on discovering ß-arrestin-biased dopamine agonists, focused on D2 partial agonists, noncatechol D1 agonists, and mixed D1/D2R dopamine receptor agonists. Here, we describe an SAR study to identify novel D1R ß-arrestin-biased ligands using A-86929, a high-affinity D1R catechol agonist, as a core scaffold to identify chemical motifs responsible for ß-arrestin-biased activity at both D1 and D2Rs. Most of the A-86929 analogs screened were G protein-biased, but none of them were exclusively arrestin-biased. Additionally, various small-fragment molecular probes displayed weak bias toward the ß-arrestin pathway. Continued in-depth SFSR (structure-functional selectivity relationship) studies informed by structure determination, molecular modeling, and mutagenesis studies will facilitate the discovery of potent and efficacious arrestin-biased dopamine receptor ligands.