Dopamine transporter trafficking: rapid response on demand

Abstract

The dopamine transporter (DAT) is a primary determinant of the concentration of dopamine in the synapse and is involved in a number of psychiatric and neurological diseases. The transporter actively takes up its physiological substrate, dopamine, when it is on the surface of the plasmalemmal membrane, but the concentration of DAT in the membrane is highly regulated by substrate. Substrates initially, and very rapidly, recruit more DAT into the membrane for greater function, but continued presence of substrate downregulates the activity of DAT and even membrane DAT content. This biphasic regulation is orchestrated by numerous signal transduction mechanisms, including a palette of protein kinases. Understanding the mechanisms of rapid regulation of DAT could provide new therapeutic strategies to improve transporter function and modulate responses to its more notorious substrates, amphetamine and methamphetamine.

Figure 1. Transporters regulate dopamine reuptake and storage in the nerve terminal

The plasmalemmal transporter, DAT, is a Na⁺, Cl⁻-dependent transporter that is normally outward-facing on the plasmalemmal membrane. Na⁺ that is transported into the terminal is transported back to the extracellular milieu by Na⁺, K⁺-ATPase. Following exocytotic release, the substrate DA diffuses to the transporter, binds to DAT and is transported into the cytosol. DA in the cytosol is transported into synaptic vesicles through the H⁺-ATPase-coupled VMAT2. AMPH (and methamphetamine), as a substrate, binds to DAT, is transported inside the terminal along with Na⁺ and Cl⁻, and depletes DA from the vesicle. However, upon dissociation of AMPH, cytosolic DA can rebind to the now inward-facing transporter; the transporter then reverses such that DA is transported back into the synapse. AMPH also inhibits the ability of DA to be transported by VMAT2 into the synaptic vesicle. The increased concentration of DA is then transported into the synapse, increasing the maximal amount of DA that can be effluxed in response to AMPH. AMPH: Amphetamine; DA: Dopamine; DAT: DA transporter; SV: Synaptic vesicles; VMAT2: Vesicular monoamine transporter 2.

Figure 2. Dopamine transporter undergoes constitutive and substrate-induced trafficking to the surface and endocytosis

DAT constitutively internalizes to and recycles from the recycling endosome and early endosome). The basal surface DAT endocytic rate is 3–5%/min. The maintenance of the basal surface DAT is PKCβ- and Rab11-dependent. DAT also undergoes time-dependent biphasic trafficking upon substrate exposure. Short-term exposure of substrates (DA or AMPH) stimulate an ultra-rapid recycling of DAT that is PKCβ dependent. Upon continuous substrate administration, DAT invaginates into membrane pits (probably when it is in the inward-facing form) and internalizes. CaMKII contributes to substrate-induced internalization. AMPH: Amphetamine; DA: Dopamine; DAT: DA transporter; CaMKII: Ca²⁺/calmodulin-dependent protein kinase II; EE: Early endosome; RE: Recycling endosome. VMAT2: Vesicular monoamine transporter 2.