Autoradiographic localization of proline uptake in excitatory hippocampal pathways

Crosstalk Among Disrupted Glutamatergic and Cholinergic Homeostasis and Inflammatory Response in Mechanisms Elicited by Proline in Astrocytes

Hyperprolinemias are inherited disorder of proline (Pro) metabolism. Patients affected may present neurological manifestations, but the mechanisms of neural excitotoxicity elicited by hyperprolinemia are far from being understood. Considering that the astrocytes are important players in neurological disorders, the aim of the present work was to study the effects 1 mM Pro on glutamatergic and inflammatory parameters in cultured astrocytes from cerebral cortex of rats, exploring some molecular mechanisms underlying the disrupted homeostasis of astrocytes exposed to this toxic Pro concentration. We showed that cortical astrocytes of rats exposed to 1 mM Pro presented significantly elevated extracellular glutamate and glutamine levels, suggesting glutamate excitotoxicity. The excess of glutamate elicited by Pro together with increased glutamate uptake and upregulated glutamine synthetase (GS) activity supported misregulated glutamate homeostasis in astrocytic cells. High Pro levels also induced production/release of pro-inflammatory cytokines TNF-α, IL-1β, and IL-6. We also evidenced misregulation of cholinergic anti-inflammatory system with increased acetylcholinesterase (AChE) activity and decreased acetylcholine (ACh) levels, contributing to the inflammatory status in Pro-treated astrocytes. Our findings highlighted a crosstalk among disrupted glutamate homeostasis, cholinergic mechanisms, and inflammatory cytokines, since ionotropic (DL-AP5 and CNQX) and metabotropic (MCPG and MPEP) glutamate antagonists were able to restore the extracellular glutamate and glutamine levels; downregulate TNFα and IL6 production/release, modulate GS and AChE activities; and restore ACh levels. Otherwise, the non-steroidal anti-inflammatory drugs nimesulide, acetylsalicylic acid, ibuprofen, and diclofenac sodium decreased the extracellular glutamate and glutamine levels, downregulated GS and AChE activities, and restored ACh levels in Pro-treated astrocytes. Altogether, our results evidence that the vulnerability of metabolic homeostasis in cortical astrocytes might have important implications in the neurotoxicity of Pro.

Vitamin D insufficiency and schizophrenia risk: evaluation of hyperprolinemia as a mediator of association

25-Hydroxyvitamin D (25(OH)D) deficits have been associated with schizophrenia susceptibility and supplementation has been recommended for those at-risk. Although the mechanism by which a deficit confers risk is unknown, vitamin D is a potent transcriptional modulator and can regulate proline dehydrogenase (PRODH) expression. PRODH maps to chromosome 22q11, a region conferring the highest known genetic risk of schizophrenia, and encodes proline oxidase, which catalyzes proline catabolism. l-Proline is a neuromodulator at glutamatergic synapses, and peripheral hyperprolinemia has been associated with decreased IQ, cognitive impairment, schizoaffective disorder, and schizophrenia. We investigated the relationship between 25(OH)D and schizophrenia, comparing fasting plasma 25(OH)D in 64 patients and 90 matched controls. We then tested for a mediating effect of hyperprolinemia on the association between 25(OH)D and schizophrenia. 25(OH)D levels were significantly lower in patients, and 25(OH)D insufficiency associated with schizophrenia (OR 2.1, adjusted p=0.044, 95% CI: 1.02-4.46). Moreover, 25(OH)D insufficient subjects had three times greater odds of hyperprolinemia than those with optimal levels (p=0.035, 95% CI: 1.08-8.91), and formal testing established that hyperprolinemia is a significantly mediating phenotype that may explain over a third of the effect of 25(OH)D insufficiency on schizophrenia risk. This study presents a mechanism by which 25(OH)D insufficiency confers risk of schizophrenia; via proline elevation due to reduced PRODH expression, and a concomitant dysregulation of neurotransmission. Although definitive causality cannot be confirmed, these findings strongly support vitamin D supplementation in patients, particularly for those with elevated proline, who may represent a large subgroup of the schizophrenia population.

PRODH polymorphisms, cortical volumes and thickness in schizophrenia

Schizophrenia is a neurodevelopmental disorder with high heritability. Several lines of evidence indicate that the PRODH gene may be related to the disorder. Therefore, our study investigates the effects of 12 polymorphisms of PRODH on schizophrenia and its phenotypes. To further evaluate the roles of the associated variants in the disorder, we have conducted magnetic resonance imaging (MRI) scans to assess cortical volumes and thicknesses. A total of 192 patients were evaluated using the Structured Clinical Interview for DSM-IV (SCID), Positive and Negative Syndrome Scale (PANSS), Calgary Depression Scale, Global Assessment of Functioning (GAF) and Clinical Global Impression (CGI) instruments. The study included 179 controls paired by age and gender. The samples were genotyped using the real-time polymerase chain reaction (PCR), restriction fragment length polymorphism (RFLP)-PCR and Sanger sequencing methods. A sample of 138 patients and 34 healthy controls underwent MRI scans. One polymorphism was associated with schizophrenia (rs2904552), with the G-allele more frequent in patients than in controls. This polymorphism is likely functional, as predicted by PolyPhen and SIFT, but it was not associated with brain morphology in our study. In summary, we report a functional PRODH variant associated with schizophrenia that may have a neurochemical impact, altering brain function, but is not responsible for the cortical reductions found in the disorder.

Proline-induced changes in acetylcholinesterase activity and gene expression in zebrafish brain: reversal by antipsychotic drugs

Hyperprolinemia is an inherited disorder of proline metabolism and hyperprolinemic patients can present neurological manifestations, such as seizures, cognitive dysfunctions, and schizoaffective disorders. However, the mechanisms related to these symptoms are still unclear. In the present study, we evaluated the in vivo and in vitro effects of proline on acetylcholinesterase (AChE) activity and gene expression in the zebrafish brain. For the in vivo studies, animals were exposed at two proline concentrations (1.5 and 3.0mM) during 1h or 7 days (short- or long-term treatments, respectively). For the in vitro assays, different proline concentrations (ranging from 3.0 to 1000 μM) were tested. Long-term proline exposures significantly increased AChE activity for both treated groups when compared to the control (34% and 39%). Moreover, the proline-induced increase on AChE activity was completely reverted by acute administration of antipsychotic drugs (haloperidol and sulpiride), as well as the changes induced in ache expression. When assessed in vitro, proline did not promote significant changes in AChE activity. Altogether, these data indicate that the enzyme responsible for the control of acetylcholine levels might be altered after proline exposure in the adult zebrafish. These findings contribute for better understanding of the pathophysiology of hyperprolinemia and might reinforce the use of the zebrafish as a complementary vertebrate model for studying inborn errors of amino acid metabolism.

Long-term proline exposure alters nucleotide catabolism and ectonucleotidase gene expression in zebrafish brain

Hyperprolinemia is an inherited disorder of proline metabolism and hyperprolinemic patients can present neurological manifestations, such as seizures cognitive dysfunctions, and psychotic disorders. However, the underlying mechanisms of these symptoms are still unclear. Since adenine nucleotides play crucial roles in neurotransmission and neuromodulation, we evaluated the in vivo and in vitro effects of proline on ectonucleotidase activities and gene expression in zebrafish brain. For the in vivo studies, animals were exposed at two proline concentrations (1.5 and 3.0 mM) during 1 h or 7 days (short- or long-term treatments, respectively). For the in vitro assays, different proline concentrations (ranging from 3.0 to 1000 μM) were tested. Short-term proline exposure did not promote significant changes on the ectonucleotidase activities and gene expression. Long-term proline exposure significantly increased ATP catabolism in both concentrations tested (14 % and 22 %, respectively), whereas ADP and AMP hydrolysis were increased only at 3.0 mM proline (21 % and 17 %, respectively) when compared to control. Moreover, the relative gene expression of enpd3 increased in both treated groups after long-term proline, whereas enptd1 increased only at 3.0 mM proline. Proline in vitro did not promote significant changes on ectonucleotidase activities. Altogether, these data indicate that the enzymes responsible for the control of extracellular nucleotides levels might be altered after proline exposure in zebrafish, contributing to better understand the pathophysiology of this disease. Moreover, such findings might facilitate the use of the zebrafish as a complementary vertebrate model for studying inborn errors of amino acid metabolism.

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.

Behavioral changes induced by long-term proline exposure are reversed by antipsychotics in zebrafish

Hyperprolinemia is an inherited disorder of proline metabolism and patients affected by this disease may present neurological manifestations, including seizures and cognitive dysfunctions. Moreover, an association between adulthood schizoaffective disorders and moderate hyperprolinemia has been reported. However, the mechanisms underlying these behavioral phenotypes still remain unclear. In the present study, we investigated the effect of proline treatments on behavioral parameters in zebrafish, such as locomotor activity, anxiety, and social interaction. Adult zebrafish (Danio rerio) were exposed to proline (1.5 and 3.0 mM) during 1h or 7 days (short- or long-term treatments, respectively). Short-term proline exposure did not promote significant changes on the behavioral parameters observed. Long-term exposure at 1.5 mM proline significantly increased the number of line crossing (47%), the total distance (29%), and the mean speed (33%) when compared to control group. A significant increase in the time spent in the upper portion of the test tank was also observed after this treatment (91%), which may be interpreted as an indicator of anxiolytic behavior. Proline at 1.5 mM also induced social interaction impairment (78%), when compared to the untreated group after long-term treatment. Moreover, these proline-induced behavioral changes in zebrafish were completely reversed by acute administration of an atypical antipsychotic drug (sulpiride), but not by a typical (haloperidol). These findings demonstrate that proline is able to induce schizophrenia-like symptoms in zebrafish, which reinforce the use of this species as a complementary vertebrate model for studying behavioral phenotypes associated with neurological dysfunctions characteristic of metabolic diseases.

Evidence that hyperprolinemia alters glutamatergic homeostasis in rat brain: neuroprotector effect of guanosine

This study investigated the effects of acute and chronic hyperprolinemia on glutamate uptake, as well as some mechanisms underlying the proline effects on glutamatergic system in rat cerebral cortex. The protective role of guanosine on effects mediated by proline was also evaluated. Results showed that acute and chronic hyperprolinemia reduced glutamate uptake, Na(+), K(+)-ATPase activity, ATP levels and increased lipoperoxidation. GLAST and GLT-1 immunocontent were increased in acute, but not in chronic hyperprolinemic rats. Our data suggest that the effects of proline on glutamate uptake may be mediated by lipid peroxidation and disruption of Na(+), K(+)-ATPase activity, but not by decreasing in glutamate transporters. This probably induces excitotoxicity and subsequent energy deficit. Guanosine was effective to prevent most of the effects promoted by proline, reinforcing its modulator role in counteracting the glutamate toxicity. However, further studies are needed to assess the modulatory effects of guanosine on experimental hyperprolinemia.

Behavioral and neurochemical effects of proline

Proline is an amino acid with an essential role for primary metabolism and physiologic functions. Hyperprolinemia results from the deficiency of specific enzymes for proline catabolism, leading to tissue accumulation of this amino acid. Hyperprolinemic patients can present neurological symptoms and brain abnormalities, whose aetiopathogenesis is poorly understood. This review addresses some of the findings obtained, mainly from animal studies, indicating that high proline levels may be associated to neuropathophysiology of some disorders. In this context, it has been suggested that energy metabolism deficit, Na(+),K(+)-ATPase, kinase creatine, oxidative stress, excitotoxicity, lipid content, as well as purinergic and cholinergic systems are involved in the effect of proline on brain damage and spatial memory deficit. The discussion focuses on the relatively low antioxidant defenses of the brain and the vulnerability of neural tissue to reactive species. This offers new perspectives for potential therapeutic strategies for this condition, which may include the early use of appropriate antioxidants as a novel adjuvant therapy, besides the usual treatment based on special diets poor in proline.

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.

Proline promotes decrease in glutamate uptake in slices of cerebral cortex and hippocampus of rats

In the present study we first investigated the in vitro and in vivo effects of proline on glutamate uptake in the cerebral cortex and hippocampus slices of rats. The action of alpha-tocopherol and/or ascorbic acid on the effects elicited by administration of proline was also evaluated. For in vitro studies, proline (30.0 microM and 1.0 mM) was added to the incubation medium. For acute administration, 29-day-old rats received one subcutaneous injection of proline (18.2 micromol/g body weight) or saline (control) and were sacrificed 1 h later. Results showed that addition of proline in the assay (in vitro studies) reduces glutamate uptake in both cerebral structures. Administration of proline (in vivo studies) reduces glutamate uptake in the cerebral cortex, but not in the hippocampal slices of rats. In another set of experiments, 22-day-old rats were pretreated for one week with daily administration of alpha-tocopherol (40 mg/kg) or ascorbic acid (100 mg/kg) or with both vitamins. Twelve hours after the last vitamins injection, rats received a single injection of proline or saline and were killed 1 h later. Pretreatment with alpha-tocopherol and/or ascorbic acid did not prevent the effect of proline administration on glutamate uptake. alpha-Tocopherol plus ascorbic acid prevented the inhibitory effect of acute hyperprolinemia on Na(+),K(+) -ATPase activity in the cerebral cortex of 29-day-old rats. The data indicate that the effect of proline on reduction of glutamate uptake and Na(+),K(+) -ATPase activity may be, at least in part, involved in the brain dysfunction observed in hyperprolinemic patients.

Proline induces alterations on nucleotide hydrolysis in synaptosomes from cerebral cortex of rats

In the present study we investigated the in vivo (acute and chronic) and in vitro effects of proline on NTPDase and 5'-nucleotidase activities in synaptosomes obtained from cerebral cortex of rats. For acute administration, 29-day-old rats received one subcutaneous injection of proline (18.2 micromol/g body weight) or an equivalent volume of 0.9% saline solution (control) and were killed 1 h later. For chronic treatment, buffered proline was injected subcutaneously into rats twice a day at 10 h intervals from the 6th to the 28th day of age. Rats were killed 12 h after the last injection. Results showed that acute and chronic proline administration provoked a reduction (25%) of ATP hydrolysis, but did not alter ADP and AMP hydrolysis. We also verified the in vitro effect of proline (3.0 microM-1.0 mM) on nucleotide hydrolysis in synaptosomes from cerebral cortex of rats. In contrast to the in vivo studies, it was not observed any statistically significant alteration on ATP, ADP and AMP hydrolysis. In conclusion, according to our results, it seems reasonable to postulate that proline administration alters the hydrolysis of ATP and probably affects the responses mediated by adenine nucleotides in the central nervous system of proline treated rats.

Proline reduces brain cytochrome c oxidase: prevention by antioxidants

In the present study, we initially investigated the in vivo (acute and chronic) and in vitro effects of proline on cytochrome c oxidase (complex IV) activity in rat cerebral cortex to test the hypothesis that proline might alter energy metabolism and that this alteration could be provoked by oxidative stress. The action of alpha-tocopherol and ascorbic acid on the effects produced by proline was also evaluated. For acute administration, 29- and 60-day-old rats received one subcutaneous injection of proline (18.2 micromol/g body weight) or an equivalent volume of 0.9% saline solution (control) and were sacrificed 1h later. For chronic treatment, proline was injected subcutaneously twice a day at 10h intervals from the 6(th) to the 28(th) day of age. Rats were sacrificed 12h (29(th)) or 31 days (60(th)) after the last injection. Results showed that acute administration of proline significantly diminished the activity of cytochrome c oxidase in the cerebral cortex of 29- and 60-day-old rats. On the other hand, chronic hyperprolinemia reduced this complex activity only on day 29, but not on the 60(th) day of life. In another set of experiments, 22-day-old rats or 53-day-old rats were pretreated for 1 week with daily intraperitoneal administration of alpha-tocopherol (40 mg/kg) and ascorbic acid (100mg/kg) or saline. Twelve hours after the last antioxidant injection, rats received a single injection of proline or saline and were killed 1h later. In parallel to chronic treatment, rats received a daily intraperitoneal injection of alpha-tocopherol and ascorbic acid from the 6(th) to the 28(th) day of life and were killed 12h after the last injection. Results showed that the pretreatment with alpha-tocopherol and ascorbic acid before acute proline administration or concomitant to chronic proline administration significantly prevented these effects. We also observed that proline (3.0 microM-1.0 mM) when added to the incubation medium (in vitro studies) did not alter cytochrome c oxidase activity. Data suggest that the inhibitory effect of proline on cytochrome c oxidase activity is possibly associated with oxidative stress and that this parameter may be involved in the brain dysfunction observed in hyperprolinemia.

The molecular genetics of the 22q11-associated schizophrenia

Schizophrenia has a strong genetic component but the mode of inheritance of the disease is complex and in all likelihood involves interaction among multiple genes and also possibly environmental or stochastic factors. A number of studies have shown that the 22q11 deletion syndrome (22q11DS) is a true genetic subtype of schizophrenia and as such may play an extremely important role in deciphering the genetic basis of schizophrenia. Microdeletions of the 22q11 locus are associated with a staggering increased risk to develop schizophrenia. The same locus has also been implicated by some linkage studies. Systematic examination of individual genes from the 1.5 Mb critical region has identified so far the PRODH and ZDHHC8 as strong candidate schizophrenia susceptibility genes from this locus. Discovery of these genes implicates neuromodulatory aminoacids and protein palmitoylation as important for disease development. Other genes, including the gene encoding for COMT, have been implicated by candidate gene approaches. It therefore appears that the 22q11-associated schizophrenia may have the characteristics of a contiguous gene syndrome, where deficiency in more than one gene contributes to the strikingly increased disease risk. Mouse models for individual candidate genes will provide the investigators with the opportunity to start understanding the function of these genes and how they may impact on schizophrenia. Mouse models that carry long-range deletions will likely capture the interactions among the culprit genes and help explain the genetic contribution of this locus to the high risk for schizophrenia. In-depth human and animal model studies of 22q11DS promise to answer critical questions relating to the devastating illness of schizophrenia, whose causes remain largely unknown.

In vivo and in vitro effects of proline on some parameters of oxidative stress in rat brain

Hyperprolinemia type II is an autosomal recessive disorder caused by the severe deficiency of delta(1)-pyrroline-5-carboxylate dehydrogenase activity leading to tissue accumulation of proline (Pro). Most patients detected so far show neurological manifestations including epilepsy and mental retardation, whose pathophysiology is not yet fully established. In the present study, we determined the in vivo and in vitro effects of Pro on some parameters of oxidative stress, namely chemiluminescence, total radical-trapping antioxidant potential (TRAP) and the activity of the antioxidant enzymes catalase (CAT), glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) from cerebral cortex of 29-day-old Wistar rats. Results showed that acute administration of Pro provoked a significant increase of chemiluminescence and a decrease of TRAP, whereas chronic administration of the metabolite did not alter these parameters. Furthermore, in vitro brain exposure to Pro resulted in increased chemiluminescence and decreased TRAP at Pro concentrations similar to those observed in tissues of hyperprolinemic patients (0.5-1.0 mM). As regards to the antioxidant enzymes, acute injection of Pro significantly decreased CAT activity and did not alter SOD and GSH-Px activities, whereas chronic Pro administration provoked a significant increase of CAT activity, a decrease of GSH-Px activity and did not modify SOD activity. Furthermore, CAT, GSH-Px and SOD activities were not affected by the presence of Pro in the incubation medium. The data indicate that Pro induces oxidative stress in vivo and in vitro, which may be involved in the brain dysfunction observed in hyperprolinemic patients.

Evidence that oxidative stress is involved in the inhibitory effect of proline on Na(+),K(+)-ATPase activity in synaptic plasma membrane of rat hippocampus

In the present study, we investigated the effect of Vitamins E and C on the inhibition of Na(+),K(+)-ATPase activity provoked by proline (Pro) administration in rat hippocampus. Five-day-old rats were pretreated for 1 week with daily i.p. administration of saline (control) or Vitamin E (40 mg/kg) and Vitamin C (100 mg/kg). Twelve hours after the last injection, animals received one single injection of Pro (12.8 micromol/g of body weight) or saline and were killed 1h later. Results showed that Na(+),K(+)-ATPase activity was decreased in the Pro-treated rats and that the pretreatment with Vitamins E and C prevented this effect. In another set of experiments, we investigated the in vitro effect of 1.0 mM Pro on Na(+),K(+)-ATPase activity from synaptic membranes of hippocampus of rats. Pro significantly inhibited (30%) Na(+),K(+)-ATPase activity. We also evaluated the effect of preincubating glutathione, trolox and N(pi)-nitro-L-arginine methyl ester (L-NAME) alone or combined with Pro on Na(+),K(+)-ATPase activity. Tested drugs did not alter Na(+),K(+)-ATPase activity, but glutathione prevented the inhibitory effect of Pro on this enzyme activity. These results suggest that the in vivo and in vitro inhibitory effect of Pro on Na(+),K(+)-ATPase activity is probably mediated by free radicals that may be involved in the neurological dysfunction found in hyperprolinemic patients.

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.

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.

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.

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.

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.

Sodium-dependent proline and glutamate uptake by hippocampal synaptosomes during postnatal development

NA(+)-dependent uptake of proline and glutamate by hippocampal synaptosomes was studied during postnatal development. At all ages from 9 days to adulthood, hippocampal synaptosomes transported proline by both a high-affinity and a low-affinity process, whereas glutamate was always transported predominantly by a high-affinity process. During the period of rapid synaptogenesis, the KT for high-affinity proline transport overshot the adult value, whereas the KT for glutamate transport increased steadily toward the adult value. The ratio of KT values for proline and glutamate was 2-3 times the adult value between 12 and 24 days of age. Although high-affinity transporters for proline and glutamate are expressed by nearly the same hippocampal pathways, they are differentially regulated during postnatal development.

Proline-glutamate interactions in the CNS

1. Crude synaptosomes (P2) and synaptosomal membranes were prepared from normal C57/B110 mouse brains and Wistar rats respectively. 2. [3H]Pro binding to mouse brain synaptic membranes was examined in the presence of competitive NMDA antagonist, MK-801, or HA-966. Conversely, the effects of l-proline on [3H]MK-801 binding were also probed. The effects of l-proline on glutamate-medicated [Ca+2]i levels were tested. 3. The authors could not detect any effect of proline on glutamate-mediated [CA+2]i levels using FURA-2 in synaptosomes or neuroblastoma cells. 4. NMDA competitive antagonists, AP-7, CPP, and CGS 19755 inhibit [3H]Pro binding to mouse brain synaptic membranes. 5. MK-801, a NMDA channel blocker, also inhibits [3H]Pro binding, but 200 mM proline is incapable of inhibiting [3H]MK-801 binding. 6. HA-966, a glycine site partial agonist inhibits [3H]Pro binding. Proline has modest effects on [3H]glycine binding.

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

The expression of a high affinity Na(+)- (and Cl-) dependent L-proline transporter (PROT) in subpopulations of putative glutamatergic pathways in rat brain raises the possibility of a specific physiological role(s) for this carrier in excitatory neurotransmission (Fremeau, R. T., Jr., Caron, M. G., and Blakely, R. D. (1992) Neuron 8, 915-926). However, the biochemical properties and regional, cellular, and subcellular distribution of the PROT protein have yet to be elucidated. Here, we document the brain-specific expression and neuronal localization of rat PROT mRNA. We also report the first identification and partial biochemical characterization of the mammalian brain PROT protein. An affinity-purified antipeptide antibody was produced that specifically recognized a single 68-kDa PROT protein on immunoblots of rat and human brain tissues. Deglycosylation of rat hippocampal membranes with peptide-N-glycosidase F reduced the apparent molecular mass of the native PROT protein from 68 to 53 kDa, the size of the primary PROT translation product determined by in vitro translation of the rat PROT cDNA in the absence of microsomes. Subcellular fractionation studies demonstrated that the PROT protein was enriched in synaptic plasma membranes but absent from postsynaptic densities. A differential distribution of PROT mRNA and protein was observed in rat striatum, suggesting that the transporter protein is synthesized in neuronal cell bodies in the cortex and exported to axon terminals in the caudate putamen. These findings warrant the consideration of a novel presynaptic regulatory role for this transporter in excitatory synaptic transmission.

NMDA receptor-mediated depolarizing action of proline on CA1 pyramidal cells

This study investigated the actions of proline on CA1 hippocampal pyramidal cells with use of slice preparations. Bath-applied L-proline first induced these cells to fire multiple orthodromic population spikes in response to a single stimulus and then blocked their response to both orthodromic and antidromic stimulation. These effects could be explained by postsynaptic depolarization followed by depolarization block. Grease-gap studies confirmed that L-proline depolarizes CA1 pyramidal cells. D-Proline was inactive in these tests. Excitatory amino acid antagonists reduced depolarizing responses to proline and N-methyl-D-aspartate (NMDA) in parallel. Mn2+ failed to attenuate proline-evoked depolarizations at concentrations that substantially inhibited synaptic transmission, but at a higher concentration it reduced responses to both proline and NMDA. These results suggest that proline depolarized CA1 pyramidal cells mainly by activating postsynaptic NMDA receptors. The neuroexcitatory and neurotoxic actions of proline in the hippocampus may contribute to the seizures and mental retardation associated with hyperprolinemia.

Molecular cloning and expression of a high affinity L-proline transporter expressed in putative glutamatergic pathways of rat brain

We have used the polymerase chain reaction (PCR) with degenerate oligonucleotides derived from two conserved regions of the norepinephrine and gamma-aminobutyric acid transporters to identify novel Na(+)-dependent transporters in rat brain. One PCR product hybridized to a 4.0 kb RNA concentrated in subpopulations of putative glutamatergic neurons including mitral cells of the olfactory bulb, pyramidal cells of layer V of the cerebral cortex, pyramidal cells of the piriform cortex, and pyramidal cells of field CA3 of the hippocampus. Transient expression of the cognate cDNA conferred Na(+)-dependent L-proline uptake in HeLa cells that was saturable (Km = 9.7 microM) and exhibited a pharmacological profile similar to that for high affinity L-proline transport in rat brain slices. The cloned transporter cDNA predicts a 637 aa protein with 12 putative transmembrane domains and exhibits 44%-45% amino acid sequence identity with other members of the emerging family of neurotransmitter transporters. These findings support a synaptic role for L-proline in specific excitatory pathways in the CNS.