L-proline and L-pipecolate induce enkephalin-sensitive currents in human embryonic kidney 293 cells transfected with the high-affinity mammalian brain L-proline transporter

In Vitro Fertilisation of Mouse Oocytes in L-Proline and L-Pipecolic Acid Improves Subsequent Development

Exposure of oocytes to specific amino acids during in vitro fertilisation (IVF) improves preimplantation embryo development. Embryos fertilised in medium with proline and its homologue pipecolic acid showed increased blastocyst formation and inner cell mass cell numbers compared to embryos fertilised in medium containing no amino acids, betaine, glycine, or histidine. The beneficial effect of proline was prevented by the addition of excess betaine, glycine, and histidine, indicating competitive inhibition of transport-mediated uptake. Expression of transporters of proline in oocytes was investigated by measuring the rate of uptake of radiolabelled proline in the presence of unlabelled amino acids. Three transporters were identified, one that was sodium-dependent, PROT (SLC6A7), and two others that were sodium-independent, PAT1 (SLC36A1) and PAT2 (SLC36A2). Immunofluorescent staining showed localisation of PROT in intracellular vesicles and limited expression in the plasma membrane, while PAT1 and PAT2 were both expressed in the plasma membrane. Proline and pipecolic acid reduced mitochondrial activity and reactive oxygen species in oocytes, and this may be responsible for their beneficial effect. Overall, our results indicate the importance of inclusion of specific amino acids in IVF medium and that consideration should be given to whether the addition of multiple amino acids prevents the action of beneficial amino acids.

Amino Acid Transporter SLC6A14 (ATB0,+) - A Target in Combined Anti-cancer Therapy

Cancer cells are characterized by quick growth and proliferation, demanding constant supply of various nutrients. Several plasma membrane transporters delivering such compounds are upregulated in cancer. Solute carrier family 6 member 14 (SLC6A14), known as amino acid transporter B^0,+ (ATB^0,+) transports all amino acids with exception of the acidic ones: aspartate and glutamate. Its malfunctioning is correlated with several pathological states and it is upregulated in solid tumors. The high expression of SLC6A14 is prognostic and unfavorable in pancreatic cancer, while in breast cancer it is expressed in estrogen receptor positive cells. As many plasma membrane transporters it resides in endoplasmic reticulum (ER) membrane after translation before further trafficking through Golgi to the cell surface. Transporter exit from ER is strictly controlled. The proper folding of SLC6A14 was shown to be controlled from the cytoplasmic side by heat shock proteins, further exit from ER and formation of coatomer II (COPII) coated vesicles depends on specific interaction with COPII cargo-recognizing subunit SEC24C, phosphorylated by kinase AKT. Inhibition of heat shock proteins, known to be upregulated in cancer, directs SLC6A14 to degradation. Targeting proteins regulating SLC6A14 trafficking is proposed as an additional pharmacological treatment of cancer.

Inactivation of the Mouse L-Proline Transporter PROT Alters Glutamatergic Synapse Biochemistry and Perturbs Behaviors Required to Respond to Environmental Changes

The endogenous neutral amino acid L-proline exhibits a variety of physiological and behavioral actions in the nervous system, highlighting the importance of accurately regulating its extracellular abundance. The L-proline transporter PROT (Slc6A7) is believed to control the spatial and temporal distribution of L-proline at glutamatergic synapses by rapid uptake of this amino acid into presynaptic terminals. Despite the importance of members of the Slc6 transporter family regulating neurotransmitter signaling and homeostasis in brain, evidence that PROT dysfunction supports risk for mental illness is lacking. Here we report the disruption of the PROT gene by homologous recombination. Mice defective in PROT displayed altered expression of glutamate transmission-related synaptic proteins in cortex and thalamus. PROT deficiency perturbed mouse behavior, such as reduced locomotor activity, decreased approach motivation and impaired memory extinction. Thus, our study demonstrates that PROT regulates behaviors that are needed to respond to environmental changes in vivo and suggests that PROT dysfunctions might contribute to mental disorders showing altered response choice following task contingency changes.

The organic osmolytes betaine and proline are transported by a shared system in early preimplantation mouse embryos

Betaine and proline protect preimplantation mouse embryos against increased osmolarity and decreased cell volume, implying that they may function as organic osmolytes. However, the transport system(s) that mediates their accumulation in fertilized eggs and early embryos was unknown, and previously identified mammalian organic osmolyte transporters could not account for their transport. Here, we report that there is a single saturable transport component shared by betaine and proline in 1-cell mouse embryos. A series of inhibitors had nearly identical effects on both betaine and proline transport by this system. In addition, K(i) values for reciprocal inhibition of betaine and proline transport were approximately 100-300 microM, similar to K(m) values ( approximately 200-300 microM) for their transport, and both had similar maximal transport rates (V(max)). The K(i) values for inhibition of betaine and proline transport by dimethylglycine were similar ( approximately 2 mM), further supporting transport of both substrates by a single transport system. Finally, betaine and proline transport each required Na(+)- and Cl(-). These data were consistent with a single, Na(+)- and Cl(-)-requiring, betaine/proline transport system in 1-cell mouse embryos. While betaine was only transported by a single saturable system, we found an additional, less conspicuous proline transport route that was betaine-insensitive, Na(+)-sensitive, and inhibited by alanine, leucine, cysteine, and methionine. Furthermore, we showed that betaine, like proline, is present in the mouse oviduct and thus could serve as a physiological substrate. Finally, accumulation of both betaine and proline increased with increasing osmolarity, consistent with a possible role as organic osmolytes in early embryos.

A channel in a transporter

1. Glutamate transporters (or excitatory amino acid transporters (EAAT)) are responsible for removing synaptically released glutamate from the extracellular space. The failure of EAAT to carry out this role will lead to excessive stimulation of glutamatergic receptors, causing excitotoxicity and cell death. 2. Glutamate is cotransported into the cell with three Na+ and one H+, followed by the counter-transport of one K+. In addition, glutamate and Na+ binding activates an uncoupled chloride conductance. Thus, glutamate transporters can function as both a transporter and an ion channel. At present, there is no clear understanding of the structural basis for the dual functions of glutamate transporters and, in the present review, we shall discuss some recent studies that have started to address this question. 3. It is possible to modulate one function of glutamate transporters without affecting the other, which suggests that the two functions have separate molecular determinants, and a number of models have been suggested to account for the dual functions of the EAAT that predict both single and dual pores for transporter function. 4. It appears that the two functions of glutamate transporters arise from separate transmembrane domains. The C-terminal region of the transporters forms the glutamate translocation domain, whereas the second transmembrane domain in the N-terminal half of the protein plays a crucial role in chloride channel function. Although the two functions arise from separate molecular determinants, the two functional domains are likely to be in close proximity. The significance of these observations will be discussed in terms of likely functional models for the transport and channel processes.

A trimeric quaternary structure is conserved in bacterial and human glutamate transporters

Neuronal and glial glutamate transporters play a central role in the termination of synaptic transmission and in extracellular glutamate homeostasis in the mammalian central nervous system. They are known to be multimers; however, the number of subunits forming a functional transporter is controversial. We studied the subunit stoichiometry of two distantly related glutamate transporters, the human glial glutamate transporter hEAAT2 and a bacterial glutamate transporter from Escherichia coli, ecgltP. Using blue native polyacrylamide gel electrophoresis, analysis of concatenated transporters, and chemical cross-linking, we demonstrated that human and prokaryotic glutamate transporters expressed in Xenopus laevis oocytes or in mammalian cells are assembled as trimers composed of three identical subunits. In an inducible mammalian cell line expressing hEAAT2 the glutamate uptake currents correlate to the amount of trimeric transporters. Overexpression and purification of ecgltP in E. coli resulted in a homogenous population of trimeric transporters that were functional after reconstitution in lipid vesicles. Our results indicate that an evolutionarily conserved trimeric quaternary structure represents the sole native and functional state of glutamate transporters.

Serotonin transporter function and pharmacology are sensitive to expression level: evidence for an endogenous regulatory factor

We express mammalian serotonin transporters (SERTs) in Xenopus oocytes by cRNA injection and measure 5-hydroxytryptamine (5-HT) transport and 5-HT-induced current at varying expression levels. Transport and current both increase sigmoidally with the amount of cRNA injected, but current requires approximately 5-fold more cRNA to elicit a half-maximal response. Western blots of SERT protein demonstrate that current, but not transport, correlates linearly with the amount of SERT on the plasma membrane. In oocytes co-injected with wild-type SERT and an inactive SERT mutant, transport is similar to SERT alone, but current is attenuated. The charge/transport ratio reports the differential sensitivity of transport and current to increasing SERT cRNA injection and mutant co-expression. Manipulations that alter the charge/transport ratio also perturb substrate and inhibitor recognition. 5-HT, d-amphetamine, cocaine, and paroxetine inhibit transport more potently at lower expression levels; however, 5-HT potency for induction of current is similar at high and low expression. Moreover, the apparent potency of cRNA for transport depends on 5-HT concentration. We postulate that SERT interacts allosterically with an endogenous factor of limited abundance to alter substrate and inhibitor potency and the balance of 5-HT transport and channel-like activity.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

A novel electrogenic amino acid transporter is activated by K+ or Na+, is alkaline pH-dependent, and is Cl--independent

A new eukaryotic nutrient amino acid transporter has been cloned from an epithelium that is exposed to high voltages and alkaline pH. The full-length cDNA encoding this novel CAATCH1 (cation-anion-activated Amino acid transporter/channel) was isolated using a polymerase chain reaction-based strategy, and its expression product in Xenopus oocytes displayed a combination of several unique, unanticipated functional properties. CAATCH1 electrophysiological properties resembled those of Na(+),Cl(-)-coupled neurotransmitter amine transporters, although CAATCH1 was cloned from a gut absorptive epithelium rather than from an excitable tissue. Amino acids such as l-proline, l-threonine, and l-methionine elicited complex current-voltage relationships in alkaline pH-dependent CAATCH1 that were reminiscent of the behavior of the dopamine, serotonin, and norepinephrine transporters (DAT, SERT, NET) in the presence of their substrates and pharmacological inhibitors such as cocaine or antidepressants. These I-V relationships indicated a combination of substrate-associated carrier current plus an independent CAATCH1-associated leakage current that could be blocked by certain amino acids. However, unlike all structurally related proteins, CAATCH1 activity is absolutely independent of Cl(-). Unlike related KAAT1, CAATCH1 possesses a methionine-inhibitable constitutive leakage current and is able to switch its narrow substrate selectivity, preferring threonine in the presence of K(+) but preferring proline in the presence of Na(+).

Differential regulation of mammalian brain-specific proline transporter by calcium and calcium-dependent protein kinases

1. This study examined the role of [Ca2+]I and Ca(2+)-dependent kinases in the modulation of high-affinity, mammalian brain-specific L-proline transporter (PROT). 2. beta-PMA (phorbol 12-myristate 13-acetate), an activator of protein kinase C (PKC), inhibits PRO uptake, and bisindolymalemide I (BIM), a potent PKC inhibitor, prevents beta-PMA inhibition. Down-regulation of PKC by chronic treatment with beta-PMA enhances PROT function indicating PROT regulation by tonic activity of PKC. 3. Thapsigargin, which increases [Ca2+]I levels by inhibiting Ca(2+)-ATPase, inhibits PROT and exhibits additive inhibition when co-treated with beta-PMA. KN-62, a Ca2+/calmodulin-dependent kinase II (CaMK II) inhibitor, but not BIM (a PKC inhibitor) prevents the inhibition by thapsigargin. These data suggest that PKC and CaMK II modulate PROT and that thapsigargin mediates its effect via CaMK II. 4. Thapsigargin raises [Ca2+]I and increases PRO-induced current on a second time scale, whereas the inhibitory effect of thapsigargin occurs only after 10 min of treatment. These data suggest that Ca2+ differentially regulate PROT: Ca2+ initially enhances PRO transport but eventually inhibits transport function through CaMK II pathway. 5. Ca(2+)-induced stimulation exemplifies the acute regulation of a neurotransmitter transporter, which may play a critical role in the profile of neurotransmitters during synaptic transmission.