The trafficking of synaptic proteins is unquestionably a major determinant of the properties of synaptic transmission. Here, we present a detailed analysis of the downregulation and intracellular trafficking of the cocaine- and amphetamine-sensitive dopamine transporter (DAT), a presynaptic plasma membrane protein responsible for the regulation of extracellular DA concentrations. Using PC12 cells stably transfected with human DAT cDNA, we observe that phorbol ester activation of protein kinase C (PKC) results in decreased transporter capacity and a parallel decrease in the amount of DAT on the cell surface that is attributable to intracellular transporter sequestration. After internalization, DAT diverges to the recycling, as opposed to the degradative, arm of the endocytic pathway. This study demonstrates, for the first time, DAT endocytosis, establishes the pathways through which DAT traffics both at steady state and in response to PKC activation, and suggests that DAT recycling is likely to occur.
The dopamine transporter (DAT) removes dopamine from the extracellular milieu and is potently inhibited by number of psychoactive drugs, including cocaine, amphetamines, and methylphenidate (Ritalin). Multiple lines of evidence demonstrate that protein kinase C (PKC) down-regulates dopamine transport, primarily by redistributing DAT from the plasma membrane to endosomal compartments, although the mechanisms facilitating transporter sequestration are not defined. Here, we demonstrate that DAT constitutively internalizes and recycles in rat pheochromocytoma (PC12) cells. Temperature blockades demonstrated basal internalization and reliance on recycling to maintain DAT cell surface levels. In contrast, recycling blockade with bafilomycin A 1 significantly decreased transferrin receptor (TfR) surface expression but had no effect on DAT surface levels, suggesting that DAT and TfR traffic via distinct endosomal mechanisms. Kinetic analyses reveal robust constitutive DAT cycling to and from the plasma membrane, independent of transporter expression levels. In contrast, phorbol ester-mediated PKC activation accelerated DAT endocytosis and attenuated transporter recycling in a manner sensitive to DAT expression levels. These data demonstrate constitutive DAT trafficking and that PKC-mediated DAT sequestration is achieved by a combination of accelerated internalization and reduced recycling. Additionally, the differential sensitivity to expression level exhibited by constitutive and regulated DAT trafficking suggests that these two processes are mediated by independent cellular mechanisms.Dopaminergic neurotransmission is fundamental to a variety of central nervous system functions, including motor control (1, 2) and cognition (3). Aberrant DA 1 neurotransmission is implicated in Parkinson's disease (4, 5) and schizophrenia (6, 7), the symptoms of which are ameliorated by increasing and decreasing DA signaling, respectively. Once released into the synapse, the primary mechanism limiting extracellular DA concentrations is presynaptic re-uptake mediated by the plasma membrane DAT. DAT belongs to the Na ϩ /Cl Ϫ -dependent transporter gene family (8, 9) and is potently inhibited by the addictive psychostimulants cocaine and amphetamine (10), making DAT a major psychostimulant target in the brain. Indeed, cocaine and amphetamines neither raise extracellular DA levels (11) nor produce hyperlocomotion (12, 13) in DAT Ϫ/Ϫ mice. A recent report also implicates DAT in nonvesicular DA release in the substantia nigra somatodendritic region (14). Hence the number of functional DATs present on the plasma membrane directly impacts dopaminergic signaling and psychostimulant efficacy.Although once considered static resident plasma membrane proteins, a growing body of evidence demonstrates that DAT surface expression is highly dynamic. The best documented example is through acute PKC activation, which down-regulates DAT and its homologues by decreasing their plasma membrane presentation (8,15,16). Moreover, DAT surface presentation is acut...
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