Vesicular and plasma membrane transporters for neurotransmitters

Abstract

The regulated exocytosis that mediates chemical signaling at synapses requires mechanisms to coordinate the immediate response to stimulation with the recycling needed to sustain release. Two general classes of transporter contribute to release, one located on synaptic vesicles that loads them with transmitter, and a second at the plasma membrane that both terminates signaling and serves to recycle transmitter for subsequent rounds of release. Originally identified as the target of psychoactive drugs, these transport systems have important roles in transmitter release, but we are only beginning to understand their contribution to synaptic transmission, plasticity, behavior, and disease. Recent work has started to provide a structural basis for their activity, to characterize their trafficking and potential for regulation. The results indicate that far from the passive target of psychoactive drugs, neurotransmitter transporters undergo regulation that contributes to synaptic plasticity.

Figure 1.

Role of plasma membrane and vesicular neurotransmitter transporters in synaptic transmission. After the exocytotic release from synaptic vesicles, neurotransmitter is transported back into the terminal by Na⁺ and Cl⁻-dependent plasma membrane transporters (PMT), thereby regenerating the vesicular pools required to sustain release. In the case of glutamate, excitatory amino acid transporters (EAATs) are generally found on cells other than those directly involved in glutamate release; most of the uptake occurs into astrocytes, mediated by EAAT1 and 2, which do not couple stoichiometrically to the flux of Cl⁻. Nonetheless, other isoforms such as EAAT3 can be expressed by neurons, although generally not at presynaptic sites or not by glutamate neurons. The glutamate taken up by glia undergoes conversion to glutamine and is then thought to recycle to neurons through the system N transporters expressed by glia and the system A transporters expressed by neurons, with conversion back to glutamate by phosphate-activated glutaminase (PAG) within neurons. Synaptic vesicles fill with neurotransmitter through a process driven by the vacuolar-type H⁺-ATPase. However, different transmitters depend on different components of the H⁺ electrochemical gradient produced by this pump. The vesicular monoamine transporter (VMAT) and closely related vesicular acetylcholine transporter depend primarily on the chemical component, ΔpH, whereas vesicular glutamate transporters (VGLUTs) depend predominantly on the membrane potential, Δψ. The entry of anions such as Cl⁻ (but also glutamate) promote the formation of ΔpH by dissipating Δψ and hence allowing the H⁺ pump to generate ΔpH, although the factors that promote Δψ have remained unexplored.

Figure 2.

Crystal structure of Gltph (PDB ID2NWX), a prokaryotic member of the SLC1 family of neurotransmitter transporters. Ribbon diagram of the Gltph trimer viewed in the membrane plane (upper panel) and from the extracellular surface (lower panel), with the protomers colored cyan, magenta, and green. L-aspartate is shown as a stick model with carbon, nitrogen, and oxygen atoms colored yellow, blue, and red, respectively. The two sodium ions identified in the structure are depicted as blue spheres. The substrates are bound at an occluded site located halfway across the membrane bilayer, near HP2, TM8, and the unwound region of TM7. (Figure courtesy of Dr. Satinder K. Singh, Department of Cellular and Molecular Physiology, Yale University School of Medicine.)

Figure 3.

Crystal structure of LeuT (PDB ID 2A65), a prokaryotic member of the SLC6 family of neurotransmitter transporters. Ribbon diagram of LeuT viewed in the plane of the membrane (upper panel) and from the extracellular surface (lower panel). TMs 1,3,6,8 are colored magenta, orange, green, and blue, respectively. L-leucine is shown as a stick model with carbon, nitrogen, and oxygen atoms colored yellow, blue, and red, respectively. The two sodium ions are depicted as cyan spheres. The substrates are bound at an occluded site at the center of the membrane bilayer, near TMs 3 and 8 and the unwound sections of TMs 1 and 6. (Figure courtesy of Dr. Satinder K. Singh, Department of Cellular and Molecular Physiology, Yale University School of Medicine.)