Mammalian brain-specific L-proline transporter. Neuronal localization of mRNA and enrichment of transporter protein in synaptic plasma membranes

MED27, SLC6A7, and MPPE1 Variants in a Complex Neurodevelopmental Disorder with Severe Dystonia

BACKGROUND

Despite advances in next generation sequencing technologies, the identification of variants of uncertain significance (VUS) can often hinder definitive diagnosis in patients with complex neurodevelopmental disorders.

OBJECTIVE

The objective of this study was to identify and characterize the underlying cause of disease in a family with two children with severe developmental delay associated with generalized dystonia and episodic status dystonicus, chorea, epilepsy, and cataracts.

METHODS

Candidate genes identified by autozygosity mapping and whole-exome sequencing were characterized using cellular and vertebrate model systems.

RESULTS

Homozygous variants were found in three candidate genes: MED27, SLC6A7, and MPPE1. Although the patients had features of MED27-related disorder, the SLC6A7 and MPPE1 variants were functionally investigated. SLC6A7 variant in vitro overexpression caused decreased proline transport as a result of reduced cell-surface expression, and zebrafish knockdown of slc6a7 exhibited developmental delay and fragile motor neuron morphology that could not be rescued by L-proline transporter-G396S RNA. Lastly, patient fibroblasts displayed reduced cell-surface expression of glycophosphatidylinositol-anchored proteins linked to MPPE1 dysfunction.

CONCLUSIONS

We report a family harboring a homozygous MED27 variant with additional loss-of-function SLC6A7 and MPPE1 gene variants, which potentially contribute to a blended phenotype caused by multilocus pathogenic variants. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

The Multifaceted Roles of Proline in Cell Behavior

Herein, we review the multifaceted roles of proline in cell biology. This peculiar cyclic imino acid is: (i) A main precursor of extracellular collagens (the most abundant human proteins), antimicrobial peptides (involved in innate immunity), salivary proteins (astringency, teeth health) and cornifins (skin permeability); (ii) an energy source for pathogenic bacteria, protozoan parasites, and metastatic cancer cells, which engage in extracellular-protein degradation to invade their host; (iii) an antistress molecule (an osmolyte and chemical chaperone) helpful against various potential harms (UV radiation, drought/salinity, heavy metals, reactive oxygen species); (iv) a neural metabotoxin associated with schizophrenia; (v) a modulator of cell signaling pathways such as the amino acid stress response and extracellular signal-related kinase pathway; (vi) an epigenetic modifier able to promote DNA and histone hypermethylation; (vii) an inducer of proliferation of stem and tumor cells; and (viii) a modulator of cell morphology and migration/invasiveness. We highlight how proline metabolism impacts beneficial tissue regeneration, but also contributes to the progression of devastating pathologies such as fibrosis and metastatic cancer.

The Lepidopteran KAAT1 and CAATCH1: Orthologs to Understand Structure-Function Relationships in Mammalian SLC6 Transporters

To the SLC6 family belong 20 human transporters that utilize the sodium electrochemical gradient to move biogenic amines, osmolytes, amino acids and related compounds into cells. They are classified into two functional groups, the Neurotransmitter transporters (NTT) and Nutrient amino acid transporters (NAT). Here we summarize how since their first cloning in 1998, the insect (Lepidopteran) Orthologs of the SLC6 family transporters have represented very important tools for investigating functional–structural relationships, mechanism of transport, ion and pH dependence and substate interaction of the mammalian (and human) counterparts.

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

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.

Comprehensive analysis of gene expression patterns in Friedreich's ataxia fibroblasts by RNA sequencing reveals altered levels of protein synthesis factors and solute carriers

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease usually caused by large homozygous expansions of GAA repeat sequences in intron 1 of the frataxin (FXN) gene. FRDA patients homozygous for GAA expansions have low FXN mRNA and protein levels when compared with heterozygous carriers or healthy controls. Frataxin is a mitochondrial protein involved in iron–sulfur cluster synthesis, and many FRDA phenotypes result from deficiencies in cellular metabolism due to lowered expression of FXN. Presently, there is no effective treatment for FRDA, and biomarkers to measure therapeutic trial outcomes and/or to gauge disease progression are lacking. Peripheral tissues, including blood cells, buccal cells and skin fibroblasts, can readily be isolated from FRDA patients and used to define molecular hallmarks of disease pathogenesis. For instance, FXN mRNA and protein levels as well as FXN GAA-repeat tract lengths are routinely determined using all of these cell types. However, because these tissues are not directly involved in disease pathogenesis, their relevance as models of the molecular aspects of the disease is yet to be decided. Herein, we conducted unbiased RNA sequencing to profile the transcriptomes of fibroblast cell lines derived from 18 FRDA patients and 17 unaffected control individuals. Bioinformatic analyses revealed significantly upregulated expression of genes encoding plasma membrane solute carrier proteins in FRDA fibroblasts. Conversely, the expression of genes encoding accessory factors and enzymes involved in cytoplasmic and mitochondrial protein synthesis was consistently decreased in FRDA fibroblasts. Finally, comparison of genes differentially expressed in FRDA fibroblasts to three previously published gene expression signatures defined for FRDA blood cells showed substantial overlap between the independent datasets, including correspondingly deficient expression of antioxidant defense genes. Together, these results indicate that gene expression profiling of cells derived from peripheral tissues can, in fact, consistently reveal novel molecular pathways of the disease. When performed on statistically meaningful sample group sizes, unbiased global profiling analyses utilizing peripheral tissues are critical for the discovery and validation of FRDA disease biomarkers.

Summary: Transcriptome profiling of Friedreich's ataxia fibroblasts by RNA sequencing reveals that this peripheral tissue can be used as a disease model for gene expression biomarker discovery.

Central mechanism of the cardiovascular responses caused by L-proline microinjected into the paraventricular nucleus of the hypothalamus in unanesthetized rats

Previously, we reported that microinjection of L-proline (L-Pro) into the paraventricular nucleus of the hypothalamus (PVN) caused vasopressin-mediated pressor responses in unanesthetized rats. In the present study, we report on the central mechanisms involved in the mediation of the cardiovascular effects caused by the microinjection of L-Pro into the PVN. Microinjection of increasing doses of L-Pro (3-100nmol/100nL) into the PVN caused dose-related pressor and bradycardic responses. No cardiovascular responses were observed after the microinjection of equimolar doses (33nmol/100nL) of its isomer D-Proline (D-Pro) or Mannitol. The PVN pretreatment with either a selective non-NMDA (NBQX) or selective NMDA (LY235959 or DL-AP7) glutamate receptor antagonists blocked the cardiovascular response to L-Pro (33nmol/100nL). The dose-effect curve for the pretreatment with increasing doses of LY235959 was located at the left in relation to the curves for NBQX and DL-AP7, showing that LY235959 is more potent than NBQX, which is more potent than DL-AP7 in inhibiting the cardiovascular response to L-Pro. The cardiovascular response to the microinjection of L-Pro into the PVN was not affected by local pretreatment with N^ω-Propyl-l-arginine (N-Propyl), a selective inhibitor of the neuronal nitric oxide synthase (nNOS), suggesting that NO does not mediate the responses to L-Pro in the PVN. In conclusion, the results suggest that ionotropic receptors in the PVN, blocked by both NMDA and non-NMDA receptor antagonists, mediate the pressor response to L-Pro that results from activation of PVN vasopressinergic magnocellular neurons and vasopressin release into the systemic circulation.

Protective effects of osmolytes in cryopreserving adherent neuroblastoma (Neuro-2a) cells

A simple method to cryopreserve adherent monolayers of neuronal cells is currently not available, but the development of this technique could facilitate numerous applications in the field of biomedical engineering, cell line development, and drug screening. However, complex tissues of some exceptional animals survive freezing in nature. These animals are known to accumulate several small molecular weight solutes prior to freezing. Following a similar strategy, we investigated the effects of osmolytes such as trehalose, proline, and sucrose as additives to the traditional cryoprotectant dimethyl sulfoxide (Me2SO) in modulating the cryopreservation outcome of mouse neuroblastoma (Neuro-2a) cells. Neuro-2a cells adhered to cell culture plates were incubated for 24 h at varying concentrations of trehalose, proline, sucrose and combinations of these compounds. Cells were cryopreserved for 24 h and cell viability post-freezing and thawing was quantified by trypan blue exclusion assay. On average, only 13.5% of adherent cells survived freezing in the presence of 10% Me2SO alone (control). Pre-incubation of cells with medium containing both trehalose and proline severely decreased cell proliferation, but increased cell recovery to about 53% of control. Furthermore, characterization using Raman microspectroscopy revealed that the addition of both trehalose and proline to 10% Me2SO substantially increased the size, and altered the nature, of ice crystals formed during freezing. Our results suggest that pre-incubation of Neuro-2a cells with trehalose and proline in combination provides cell protection along with alterations of ice structure in order to increase cell survival post-freezing.

Amino acid secondary transporters: toward a common transport mechanism

Solute carriers (SLC) that transport amino acids are key players in health and diseases in humans. Their prokaryotic relatives are often involved in essential physiological processes in microorganisms, e.g. in homeostasis and acidic/osmotic stress response. High-resolution X-ray structures of the sequence-unrelated amino acid transporters unraveled a striking structural similarity between carriers, which were formerly assigned to different families. The highly conserved fold is characterized by two inverted structural repeats of five transmembrane helices each and indicates common mechanistic transport concepts if not an evolutionary link among a large number of amino acid transporters. Therefore, these transporters are classified now into the structural amino acid-polyamine-organocation superfamily (APCS). The APCS includes among others the mammalian SLC6 transporters and the heterodimeric SLC7/SLC3 transporters. However, it has to be noted that the APCS is not limited entirely to amino acid transporters but contains also transporters for, e.g. amino acid derivatives and sugars. For instance, the betaine-choline-carnitine transporter family of bacterial activity-regulated Na(+)- and H(+)-coupled symporters for glycine betaine and choline is also part of this second largest structural superfamily. The APCS fold provides different possibilities to transport the same amino acid. Arginine can be transported by an H(+)-coupled symport or by antiport mechanism in exchange against agmatine for example. The convergence of the mechanistic concept of transport under comparable physiological conditions allows speculating if structurally unexplored amino acid transporters, e.g. the members of the SLC36 and SLC38 family, belong to the APCS, too. In the kidney, which is an organ that depends critically on the regulated amino acid transport, these different SLC transporters have to work together to account for proper function. Here, we will summarize the basic concepts of Na(+)- and H(+)-coupled amino acid symport and amino acid-product antiport in the light of the respective physiological requirements.

The solute carrier 6 family of transporters

The solute carrier 6 (SLC6) family of the human genome comprises transporters for neurotransmitters, amino acids, osmolytes and energy metabolites. Members of this family play critical roles in neurotransmission, cellular and whole body homeostasis. Malfunction or altered expression of these transporters is associated with a variety of diseases. Pharmacological inhibition of the neurotransmitter transporters in this family is an important strategy in the management of neurological and psychiatric disorders. This review provides an overview of the biochemical and pharmacological properties of the SLC6 family transporters.

Cardiovascular effects of the microinjection of L-proline into the third ventricle or the paraventricular nucleus of the hypothalamus in unanesthetized rats

We investigated the cardiovascular effects of the microinjection of L-proline (L-Pro) into the third ventricle (3V) and its peripheral mechanisms. Different doses of L-Pro into the 3V caused dose-related pressor and bradycardiac responses. The pressor response to L-Pro injected into the 3V was potentiated by intravenous pretreatment with the ganglion blocker pentolinium (5 mg/kg), thus excluding any significant involvement of the sympathetic nervous system. Because the response to the microinjection of L-Pro into the 3V was blocked by intravenous pretreatment with the V1-vasopressin receptor antagonist dTyr(CH(2) )(5) (Me)AVP (50μg/kg), it is suggested that these cardiovascular responses are mediated by a vasopressin release. The pressor response to the microinjection of L-Pro into the 3V was found to be mediated by circulating vasopressin, so, given that the paraventricular nucleus of the hypothalamus (PVN) is readily accessible from the 3V, we investigated whether the PVN could be a site of action for the L-Pro microinjected in the 3V. The microinjection of L-Pro (0.033 μmoles/0.1 μl) into the PVN caused cardiovascular responses similar to those of injection of the 3V and were also shown to be mediated by vasopressin release. In conclusion, these results show that the microinjection of L-Pro into the 3V causes pressor and bradycardiac responses that could involve stimulation of the magnocellular cells of the PVN and release of vasopressin into the systemic circulation. Also, because the microinjection of L-Pro into the PVN caused a pressor response, this is the first evidence of cardiovascular effects caused by its injection in a supramedullary structure.

Evidence for association of hyperprolinemia with schizophrenia and a measure of clinical outcome

There are multiple genetic links between schizophrenia and a deficit of proline dehydrogenase (PRODH) enzyme activity. However, reports testing for an association of schizophrenia with the resulting proline elevation have been conflicting. The objectives of this study were to investigate whether hyperprolinemia is associated with schizophrenia, and to measure the relationship between plasma proline, and clinical features and symptoms of schizophrenia. We performed a cross-sectional case-control study, comparing fasting plasma proline in 90 control subjects and 64 schizophrenic patients and testing for association of mild to moderate hyperprolinemia with schizophrenia. As secondary analyses, the relationship between hyperprolinemia and five measures of clinical onset, symptoms and outcome were investigated. Patients had significantly higher plasma proline than matched controls (p<0.0001), and categorical analysis of gender adjusted hyperprolinemia showed a significant association with schizophrenia (OR 6.15, p=0.0003). Hyperprolinemic patients were significantly older at their first hospitalization (p=0.015 following correction for multiple testing). While plasma proline level was not related to total, positive or negative symptoms, hyperprolinemic status had a significant effect on length of hospital stay (p=0.005), following adjustment for race, BPRS score, and cross-sectional time from admission to proline measurement. Mild to moderate hyperprolinemia is a significant risk factor for schizophrenia, and may represent an intermediate phenotype in the disease. Hyperprolinemic patients have a significantly later age of first psychiatric hospitalization, suggestive of later onset, and hospital stays 46% longer than non-hyperprolinemic subjects. These findings have implications in the etiology of schizophrenia, and for the clinical management of these patients.

A new association between polymorphisms of the SLC6A7 gene in the chromosome 5q31-32 region and asthma

The human chromosomal 5q31-33 region has been implicated as a susceptibility locus for several immune-mediated diseases including asthma in several populations. Recently, the extraneuronal GABAergic system has been implicated as a new link to airway obstruction in asthma. In addition, the SLC6A7 gene, which is positioned at 5q31-32 and encodes the transporter for an excitatory neurotransmitter of L-proline, has never been studied for its association with asthma. In this study, resequencing of all exon, promoter region (2 kb), and exon-intron boundary regions in the SLC6A7 gene found a total of 33 single nucleotide polymorphisms (SNPs) in 24 Korean asthmatic patients. After the initial SNP survey, a total of 17 common SNPs with minor allele frequency (MAF) over 10% were genotyped in 498 asthmatic patients and 303 normal controls. Logistic analyses revealed significant associations between genetic variants of the SLC6A7 gene and asthma (P-value up to 6.0 x 10(-4); P(corr) value up to 0.009). In further regression analyses, minor alleles of intronic +11431T>C, +12213C>T and +12927A>G in linkage disequilibrium block 2 and +20113T>C in 3'UTR significantly increased the bronchodilator response in asthmatics (P-value of recessive model up to 0.008; which are not significant after multiple correction). Therefore, our findings suggest that SLC6A7 could be a susceptible gene for asthma.

Functional consequences of PRODH missense mutations

PRODH maps to 22q11 in the region deleted in the velocardiofacial syndrome/DiGeorge syndrome (VCFS/DGS) and encodes proline oxidase (POX), a mitochondrial inner-membrane enzyme that catalyzes the first step in the proline degradation pathway. At least 16 PRODH missense mutations have been identified in studies of type I hyperprolinemia (HPI) and schizophrenia, 10 of which are present at polymorphic frequencies. The functional consequences of these missense mutations have been inferred by evolutionary conservation, but none have been tested directly. Here, we report the effects of these mutations on POX activity. We find that four alleles (R185Q, L289M, A455S, and A472T) result in mild (<30%), six (Q19P, A167V, R185W, D426N, V427M, and R431H) in moderate (30%-70%), and five (P406L, L441P, R453C, T466M, and Q521E) in severe (>70%) reduction in POX activity, whereas one (Q521R) increases POX activity. The POX encoded by one severe allele (T466M) shows in vitro responsiveness to high cofactor (flavin adenine dinucleotide) concentrations. Although there is limited information on plasma proline levels in individuals of known PRODH genotype, extant data suggest that severe hyperprolinemia (>800 microM) occurs in individuals with large deletions and/or PRODH missense mutations with the most-severe effect on function (L441P and R453C), whereas modest hyperprolinemia (300-500 microM) is associated with PRODH alleles with a moderate reduction in activity. Interestingly, three of the four alleles associated with or found in schizophrenia (V427M, L441P, and R453C) resulted in severe reduction of POX activity and hyperprolinemia. These observations plus the high degree of polymorphism at the PRODH locus are consistent with the hypothesis that reduction in POX function is a risk factor for schizophrenia.

Chronic hyperprolinemia provokes a memory deficit in the Morris water maze task

In the present study we investigated the effect of chronic proline (Pro) administration on rat performance in the Morris water maze task. Rats received s.c. injections of Pro twice a day at 8 h intervals from the 6th to the 28th days of age and equivalent volume of 0.9% saline solution (control). On the 60th day of life, rats were subjected to the water maze task. Results showed that chronic Pro administration provokes impairment on spatial learning, as shown by the increase of latency in acquisition and retention and by a reduced efficiency to find the platform position in the working memory test. Present results suggest that hyperprolininemia causes cognitive dysfunction and might be relevant to explain, at least in part, the neurological dysfunction associated with hyperprolinemia.

Transport of proline and hydroxyproline by the neutral amino-acid exchanger ASCT1

ASCT1 is a member of the glutamate transporter superfamily cloned from human brain and characterized as a Na(+)-dependent neutral amino-acid exchanger, which displays substrate-induced chloride-channel activity and mediates concentrative transport of alanine. Initial studies in ASCT1-expressing Xenopus laevis oocytes showed that proline did not elicit measurable currents, in contrast to what occurred with alanine, serine or cysteine, suggesting that proline was not an ASCT1 substrate, although it induced the release of alanine from preloaded oocytes. Here, we have studied the uptake of proline and hydroxyproline by ASCT1-expressing oocytes in order to investigate the ability of ASCT1 to translocate these imino acids. The results demonstrate ASCT1-mediated proline transport that is Na(+)-dependent, saturable, inhibited by the reported ASCT1 substrates as well as by hydroxyproline and can drive the imino acid against its concentration gradient. The apparent kinetic constants for the transport of alanine and the imino acids, obtained with oocytes from the same batch, showed maximal transport rate for proline and hydroxyproline to be half of that for alanine. However, K(0.5) for proline was 704 +/- 86 microM, about three times higher than alanine K(0.5) (203.3 +/- 36.4 microM), whereas hydroxyproline K(0.5) was 33.2 +/- 4.3 microM, indicating that the hydroxylation on carbon 4 of proline strongly increases the affinity of ASCT1 for this proline derivative. In summary, the present work demonstrates for the first time the ability of ASCT1 to transport proline and hydroxyproline.

Proline reduces creatine kinase activity in the brain cortex of rats

Type II hyperprolinemia is an inherited disorder caused by a deficiency of delta 1-pyrroline-5-carboxilic acid dehydrogenase, whose biochemical hallmark is proline accumulation in plasma and tissues. Although neurological symptoms occur in most patients, the neurotoxicity of proline is still controversial. The main objective of the present study was to investigate the effect of acute and chronic administration of proline on creatine kinase activity of brain cortex of Wistar rats. Acute treatment was performed by subcutaneous administration of one injection of proline to 22-day-old rats. For chronic treatment, proline was administered twice a day from the 6th to the 21st postpartum day. The results showed that creatine kinase activity was significantly inhibited in the brain cortex of rats subjected to acute proline administration. In contrast, this activity was increased in animals subjected to chronic administration. We also measured the in vitro effect of proline on creatine kinase activity in cerebral cortex of 22-day-old nontreated rats. Proline significantly inhibited creatine kinase activity. Considering the importance of creatine kinase forthe maintenance of energy homeostasis in the brain, it is conceivable that an alteration of this enzyme activity in the brain may be one of the mechanisms by which proline might be neurotoxic.

Lysosomal amino acid transporter LYAAT-1 in the rat central nervous system: an in situ hybridization and immunohistochemical study

A first mammalian lysosomal transporter (LYAAT-1) was recently identified and functionally characterized. Preliminary immunocytochemical data revealed that LYAAT-1 localizes to lysosomes in some neurons. In order to determine whether it is expressed in specific neuron populations and other cell types, and to confirm whether it is localized at the membrane of lysosomes, we used in situ hybridization and immunohistochemistry methods in adult rat central nervous system (CNS). We found that LYAAT-1 is expressed in most areas of the CNS, specifically in neurons, but also in choroid plexus and ependymal epithelium cells. LYAAT-1-IR (immunoreactivity) levels varied among different neuroanatomical structures but were present in neurons independently of the neurotransmitter used (glutamate, GABA, acetylcholine, noradrenaline, serotonin, or glycine). Light and confocal microscopy demonstrated that LYAAT-1 and the lysosomal marker cathepsin D colocalized throughout the brain and electron microscopy showed that LYAAT-1-IR was associated with lysosomal membranes. In addition, LYAAT-1-IR was also found associated with other membranes belonging to the Golgi apparatus and lateral saccules and less frequently with multivesicular bodies, endoplasmic reticulum, and occasionally with the plasma membrane. The localization of LYAAT-1 at the lysosomal membrane is consistent with the view that it mediates amino acid efflux from lysosomes. Furthermore, its cell expression pattern suggests that it may contribute to specialized cellular function in the rat CNS such as neuronal metabolism, neurotransmission, and control of brain amino acid homeostasis.

Effect of proline on creatine kinase activity in rat brain

Type II Hyperprolinemia is an inherited disorder caused by a deficiency of delta1-pyrroline-5-carboxilic acid dehydrogenase, whose biochemical hallmark is proline accumulation in plasma and tissues. Although neurologic symptoms occur in most patients, the neurotoxicity of proline is still controversial. The main objective of this study was to investigate the effect of acute and chronic administration of proline on creatine kinase activity in the homogenates of cerebellum and midbrain from Wistar rats. Acute treatment was performed by subcutaneous administration of one injection of proline to 22-day-old rats. For chronic treatment, proline was administered four times a day from the 6th to the 21st postpartum day. The results showed that creatine kinase activity was significantly inhibited in the cerebellum and midbrain of rats subjected to acute proline administration. In contrast, this activity was increased in animals subjected to chronic administration. We also measured the in vitro effect of proline on creatine kinase activity in the same cerebral structures of 22-day-old nontreated rats. Proline significantly inhibited creatine kinase activity. Considering the importance of creatine kinase for the maintenance of energy homeostasis in the brain, it is conceivable that an alteration of this enzyme activity in the brain may be one of the mechanisms by which proline might be neurotoxic.

Proline induces oxidative stress in cerebral cortex of rats

In the present study we investigated the in vivo and in vitro effects of proline on some parameters of oxidative stress, such as chemiluminescence, total radical-trapping antioxidant potential (TRAP) and the activity of the antioxidant enzymes catalase, glutathione peroxidase and superoxide dismutase in rat cerebral cortex. Ten-day-old rats received one subcutaneous injection of proline (12.8 micromol/g body weight), while control rats received saline in the same volumes. The animals were killed 1h after injection, the cerebral cortex was isolated and the assays immediately carried out. For the in vitro studies, homogenates from cerebral cortex of 10-day-old untreated rats were incubated for 1h at 37 degrees C with various concentrations of proline (3.0 microM-1.0mM). Results showed that proline-treated rats presented a decrease of TRAP (30%) and an increase of chemiluminescence (78%). In contrast, the activities of catalase, glutathione peroxidase and superoxide dismutase were not modified by proline acute treatment. Furthermore, the presence of proline in the medium increased chemiluminescence, decreased TRAP and the activity of superoxide dismutase at proline concentrations similar to those observed in tissues of hyperprolinemic patients (0.5-1.0mM). However, catalase and glutathione peroxidase activities were not affected by the presence of proline in the medium. The results indicate that proline induces oxidative stress in the brain, which may be related, at least in part, to the neurological dysfunction observed in hyperprolinemia.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Molecular cloning and functional expression of a proline transporter from Manduca sexta

We report the molecular cloning of a L-proline transporter, MasPROT cDNA and its splice variants MasPROT.16 and MasPROT.2 from the central nervous system of Manduca sexta. Sequence analysis revealed that MasPROT belongs to a family of high affinity Na+/Cl- dependent neurotransmitter transporters. The deduced amino acid (aa) sequence of 556 aa having an estimated molecular mass of 58.9 kDa is predicted to have 12 putative transmembrane domains (TMD) and a characteristic large extracellular loop between TMD3 and TMD4. Sequence comparison to other members of the family indicates that it falls into the glycine-proline transporter subfamily. Transiently expressed MasPROT cDNA in Xenopus oocytes exclusively transported proline. Northern analysis shows that it is expressed predominantly in central nervous system, however, low levels are present in midgut, hindgut and Malpighian tubules. Two mRNA transcripts of sizes 3.6 and 8 Kb were found in all tissues except hindgut, where only a smaller transcript exists. RT-PCR and Southern blot analysis revealed the presence of MasPROT transcripts in flight muscles but not in leg muscles. Our preliminary data suggests that this transporter is an insect homologue of mammalian proline transporters. MasPROT.16 is a short splice variant encoding for 174 amino acids and shares 138 amino acids from the N terminus of MasPROT. MasPROT.2 is a long splice variant that contains six introns that coincide precisely with the previously mapped exon/intron boundaries of the members of this superfamily.

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.

Proline administration decreases Na+,K+-ATPase activity in the synaptic plasma membrane from cerebral cortex of rats

Buffered proline was injected subcutaneously into rats twice a day at 8 h intervals from the 6th to the 28th day of age. Control rats received saline in the same volumes. The animals were weighed and killed by decapitation 12 h after the last injection. Cerebral cortex was used for the determination of Na+,K+-ATPase and Mg2+-ATPase activities. Body, whole brain and cortical weights were similar in the two groups. Na+,K+-ATPase activity was significantly reduced (by 20%) in membranes from the proline-treated group compared to the controls, whereas Mg2+-ATPase activity was not affected by proline. In another set of experiments, synaptic plasma membranes were prepared from cerebral cortex of 29-day-old rats and incubated with proline at final concentrations ranging from 0.1 to 2.0 mM. Na+,K+-ATPase activity, but not Mg2+-ATPase activity, was inhibited by 20-30%. Since proline concentrations in plasma of chronically treated rats and of type 11 hyperprolinemic children are of the same order of magnitude as those tested in vitro, the results suggest that reduction of Na+,K+-ATPase activity may contribute to the neurological dysfunction found in some patients affected by type II hyperprolinemia.

Membrane trafficking of neurotransmitter transporters in the regulation of synaptic transmission

Many psychoactive drugs influence the transport of neurotransmitters across biological membranes, suggesting that the physiological regulation of neurotransmitter transport might contribute to normal and perhaps abnormal behaviour. Over the past few years, molecular characterization of the neurotransmitter transporters has enabled investigation of their subcellular location and regulation. The analysis of location suggests that membrane trafficking has an important role in the normal function of these proteins. One of the major regulatory mechanisms also involves changes in localization that might contribute to synaptic plasticity. This article discusses recent work on the membrane trafficking of neurotransmitter transporters and its role in regulating their activity.

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

The high-affinity mammalian brain L-proline transporter (PROT) belongs to the GAT1 gene family, which includes Na- and Cl-dependent plasma membrane carriers for neurotransmitters, osmolites, and metabolites. These transporters couple substrate flux to transmembrane electrochemical gradients, particularly the Na gradient. In the nervous system, transporters clear synapses and help to replenish transmitters in nerve terminals. The localization of PROT to specific excitatory terminals in rat forebrain suggests a role for this carrier in excitatory transmission (). We investigated the voltage regulation and electrogenicity of this novel transporter, using human embryonic kidney (HEK) 293 cells stably transfected with rat PROT cDNA. In physiological solutions between -140 and -40 mV, L-proline (PRO) and its six-member ring congener L-pipecolate (PIP) induced inward current. The current-voltage relationship and the variance of current fluctuations were similar for PRO- and PIP-induced current, and the ratio of induced variance to the mean current ranged from 20 to 60 fA. Des-Tyr-Leu-enkephalin (GGFL), a competitive peptide inhibitor of PROT, reduced the rat PROT-associated current to control levels. GGFL alone did not elicit currents, and the GGFL-sensitive substrate-induced current was absent in nontransfected cells. Finally, GGFL inhibited PROT-mediated transport only when applied to the extracellular face of PROT. These data suggest that (1) PROT uptake is electrogenic, (2) individual transporter currents are voltage-independent, and (3) GGFL is a nonsubstrate inhibitor that interacts either with an extracellular domain of PROT or in an externally accessible pore.

Molecular enzymology of mammalian Delta1-pyrroline-5-carboxylate synthase. Alternative splice donor utilization generates isoforms with different sensitivity to ornithine inhibition

Delta1-Pyrroline-5-carboxylate synthase (P5CS; EC not assigned), a mitochondrial inner membrane, ATP- and NADPH-dependent, bifunctional enzyme, catalyzes the reduction of glutamate to Delta1-pyrroline-5-carboxylate, a critical step in the de novo biosynthesis of proline and ornithine. We utilized published plant P5CS sequence to search the expressed sequence tag data base and cloned two full-length human P5CS cDNAs differing in length by 6 base pairs (bp) in the open reading frame. The short cDNA has a 2379-bp open reading frame encoding a protein of 793 residues; the long cDNA, generated by "exon sliding," a form of alternative splicing, contains an additional 6-bp insert following bp +711 of the short form resulting in inclusion of two additional amino acids in the region predicted to be the gamma-glutamyl kinase active site of P5CS. The long form predominates in all tissues examined except gut. We also isolated the corresponding long and short murine P5CS transcripts. To confirm the identity of the putative P5CS cDNAs, we expressed both human forms in gamma-glutamyl kinase- and gamma-glutamyl phosphate reductase-deficient strains of Saccharomyces cerevisiae and showed that they conferred the proline prototrophy. Additionally, we found expression of the murine putative P5CS cDNAs conferred proline prototrophy to P5CS-deficient Chinese hamster ovary cells (CHO-K1). We utilized stable CHO-K1 cell transformants to compare the biochemical characteristics of the long and short murine P5CS isoforms. We found that both confer P5CS activity and that the short isoform is inhibited by L-ornithine with a Ki of approximately 0.25 mM. Surprisingly, the long isoform is insensitive to ornithine inhibition. Thus, the two amino acid insert in the long isoform abolishes feedback inhibition of P5CS activity by L-ornithine.

The mammalian brain high-affinity L-proline transporter is enriched preferentially in synaptic vesicles in a subpopulation of excitatory nerve terminals in rat forebrain

The expression of a brain-specific high-affinity Na+-dependent (and Cl--dependent) L-proline transporter (PROT) in subpopulations of putative glutamatergic neurons in mammalian brain suggests a physiological role for this carrier in excitatory neurotransmission (). 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.

Developmental expression of the neurotransmitter transporter NTT4

Neurotransmitter transporters are involved in termination of the synaptic neurotransmission and are implicated as the sites of action of antidepressant medicines and illicit drugs. In addition to their function in neurotransmission, neurotransmitter transporters play a key role in neuroregulation and brain development. In this report, the developmental distribution of the "orphan" transporter NTT4, whose substrate has not yet been shown, is described. Immunohistochemical studies have previously shown NTT4 to be specifically and widely localized to the central nervous system. In this report, the distribution of NTT4 in brain areas enriched in glutamatergic and gamma-aminobutyric acid-ergic innervations is further substantiated. NTT4 is detected beginning at E18 in various parts of the rat brain, including the cerebral cortex, fimbria hippocampi, fornix, lateral lemniscus, anterior commissure, and spinal cord. At E18, strong immunoreactivity of NTT4 is observed in the cortical subplate and marginal layers that later develops into the fimbria hippocampi, and at P22, the expression of NTT4 in the hippocampal formation reaches the mature form. The expression of NTT4 in the spinal cord begins at E18 in the ventral white matter. Heavy staining for NTT4 is observed in the substantia nigra since birth and through all time points examined. Transient immunoreactivity is observed in the inferior colliculus, reaching maximal expression at P10, whereas the superior colliculus commences to express NTT4 only after this time point. The globus pallidus is highly stained after birth, and the caudate putamen shows strong staining for NTT4 only at P22. In the adult rat brain, NTT4 is strongly expressed in the olfactory bulb, cerebral cortex, striatum, hippocampus, thalamus, substantia nigra, pontine nucleus, cerebellum, and spinal cord. The developmental distribution of NTT4 suggests involvement in central nervous system maturation.

Localization of the brain-specific high-affinity l-proline transporter in cultured hippocampal neurons: molecular heterogeneity of synaptic terminals

The expression of a brain-specific, high-affinity Na+-(and Cl--)dependent l-proline transporter in subpopulations of putative glutamatergic pathways in mammalian brain suggests a physiological role for this carrier in excitatory neurotransmission (Fremeau et al. , Neuron 8: 915-926, 1992). To assess further the cell-type and subcellular localization of PROT, we examined its distribution in low-density cultures of embryonic rat hippocampus. PROT immunoreactivity was detected beginning at 8 days in culture in a highly punctate pattern localizing to a subset of synaptic terminals. PROT was not detected at GABAergic terminals but was specifically localized to a subset of excitatory nerve terminals. PROT-labeled terminals showed partial apposition to AMPA-type and NMDA-type glutamate receptor clusters. Immunolabeling of isolated neurons grown in microisland cultures revealed that PROT was expressed by 60% of cultured hippocampal neurons. Individual microisland cultures were immunopositive for either PROT or glutamic acid decarboxylase, but never both. In the expressing pyramidal neurons, PROT was targeted to all presynaptic terminals. These findings indicate that PROT contributes to the molecular heterogeneity of glutamatergic terminals and suggest a novel presynaptic regulatory role for PROT in excitatory transmission at specific glutamatergic synapses.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Proline-induced inhibition of glutamate release in hippocampal area CA1

Concentrations of proline typical of human CSF have been shown to potentiate transmission at Schaffer collateral-commissural synapses on CA1 pyramidal cells of the rat hippocampus. This study tested the hypothesis that proline enhances excitatory synaptic transmission by increasing glutamate release. Two concentrations of proline were used: a concentration typical of normal human CSF (3 microM) and a concentration typical of CSF in persons with the genetic disorder hyperprolinemia type II (30 microM). Continuous exposure of hippocampal slices to either concentration of proline potentiated Schaffer collateral-commissural synaptic transmission. Proline shifted the plot of field EPSP slope against fiber volley amplitude upward. Contrary to the original hypothesis, neither concentration of proline reduced paired-pulse facilitation; 30 microM proline enhanced paired-pulse facilitation, whereas 3 microM proline had no effect. In line with its enhancement of paired-pulse facilitation, 30 microM proline reduced both the K+-evoked release of glutamate and aspartate from CA1 slices and the release of glutamate and aspartate from CA1 synaptosomes evoked by 4-aminopyridine. These results suggest that the proline-induced potentiation of Schaffer collateral-commissural synaptic transmission probably involves a postsynaptic, rather than a presynaptic, mechanism. Concentrations of proline normally found in human CSF little affect glutamate release. However, proline-induced inhibition of glutamate release may contribute to the neuropsychiatric disorders associated with hyperprolinemia type II.

Proline-induced potentiation of glutamate transmission

The amino acid proline has long been suspected to serve as a modulator of synaptic transmission in the mammalian brain, but no such function has been identified. The selective expression of high affinity proline transport by a subset of glutamate pathways suggested that proline might play a role in synaptic transmission at these sites. This idea was tested with use of one such pathway, the Schaffer collateral-commissural projection to CA1 pyramidal cells of the rat hippocampus. Proline enhanced the initial slope of the field EPSP without affecting axonal excitability or the magnitude of paired-pulse facilitation. Proline-induced potentiation far outlasted the period of proline application and required the activation of NMDA receptors. Proline enhanced Schaffer collateral-commissural synaptic transmission even when the connections between areas CA1 and CA3 had been interrupted. Potentiation was observed with a proline concentration normally present in human CSF (3 microM). A concentration typical of CSF in persons with the genetic disorder hyperprolinemia type II (30 microM) produced a somewhat greater effect. Occlusion experiments suggested that proline-induced potentiation and tetanus-induced long-term potentiation utilize largely distinct transduction mechanisms. Proline-induced potentiation could be blocked by a prior high frequency stimulus, whether or not the stimulus evoked long-term potentiation. These results suggest that endogenous extracellular proline regulates the basal function of some glutamate synapses by maintaining them in a partially potentiated state. They may also facilitate understanding of the seizures and/or mental retardation associated with genetic disorders of proline metabolism.