The Mammalian Brain High-Affinity l-Proline Transporter Is Enriched Preferentially in Synaptic Vesicles in a Subpopulation of Excitatory Nerve Terminals in Rat Forebrain

Abstract

The expression of a brain-specific high-affinity Na ϩ -dependent (and Cl Ϫ -dependent) L-proline transporter (PROT) in subpopulations of putative glutamatergic neurons in mammalian brain suggests a physiological role for this carrier in excitatory neurotransmission (Fremeau et al., 1992). To gain insights into potential sites where PROT may function, we used a C-terminal domain antipeptide antibody to determine the regional distribution and subcellular localization of PROT in rat forebrain. PROT immunoreactivity was seen in processes having a regional light microscopic distribution comparable to that of known glutamatergic projections within the cortex, caudate putamen nucleus (CPN), hippocampal formation, and other forebrain regions. In all regions examined by electron microscopy (cortex, CPN, and the stratum oriens of CA1), PROT labeling was observed primarily within subpopulations of axon terminals forming asymmetric excitatory-type synapses. Immunogold labeling for PROT was detected in close contact with membranes of small synaptic vesicles (SSVs) and more rarely with the plasma membrane in these axon terminals. Subcellular fractionation studies confirmed the preferential distribution of PROT to synaptic vesicles. The topology of PROT in synaptic vesicles was found to be inverted with respect to the plasma membrane, suggesting that PROT-containing vesicles are generated by a process involving endocytosis from the plasma membrane. Because PROT lacks any of the known characteristics of other vesicular transporters, these results suggest that certain excitatory terminals have a reserve pool of PROT associated with SSVs. The delivery of PROT to the plasma membrane by exocytosis could play a critical role in the plasticity of certain glutamatergic pathways.

Evidence for a Primary Endocytic Vesicle Involved in Synaptic Vesicle Biogenesis

“…We also looked for the presence of the plasma membrane dopamine transporter, which mediates the high affinity, sodiumdependent uptake of monoamines across the plasma membrane. The dopamine transporter is excluded from synaptic vesicles in rat brain and from synaptic vesicles in PC12 cells following exogenous expression (21,28). The dopamine transporter was enriched at approximately 15% glycerol, the same concentration of glycerol as GLUT1, GLUT3, and GLUT4 (data not shown).…”

Na+, Cl−, and pH Dependence of the Human Choline Transporter (hCHT) inXenopusOocytes: The Proton Inactivation Hypothesis of hCHT in Synaptic Vesicles

“…Acidic inactivation of CHTs may provide the mechanism that prevents such leakage from synaptic vesicles in cholinergic neurons. The distribution of transporters on synaptic vesicles has also been observed for glycine transporters (Geerlings et al, 2001) and L-proline transporters (Renick et al, 1999), suggesting that evolution of a tight pH regulation system to allow use of the vesicles to support activitydependent trafficking (Ferguson et al, 2003).…”

Estudos traducionais de neuropsiquiatria e esquizofrenia: modelos animais genéticos e de neurodesenvolvimento

“…Selective deletion of PRODH in mice has been shown to lead to deficits in sensorimotor gating, with decreased glutamate, aspartate and GABA levels in the frontal cortex 85 . Proline has been implicated in modulation of neurotransmission especially in the glutamatergic system, which could lead to the psychotic symptoms observed in patients [86][87][88] . 22q11DS mouse models present with a loss of glutamatergic synapses, decreased spine density, decreased CA1 pyramidal neuron complexity and reduced neurogenesis in the subventricular zone.…”