Synaptic localisation of agmatinase in rat cerebral cortex revealed by virtual pre-embedding

As Verified with the Aid of Biotinylated Spermine, the Brain Cannot Take up Polyamines from the Bloodstream Leaving It Solely Dependent on Local Biosynthesis

The importance of polyamines (PAs) for the central nervous system (CNS) is well known. Less clear, however, is where PAs in the brain are derived from. Principally, there are three possibilities: (i) intake by nutrition, release into the bloodstream, and subsequent uptake from CNS capillaries, (ii) production by parenchymatous organs, such as the liver, and again uptake from CNS capillaries, and (iii) uptake of precursors, such as arginine, from the blood and subsequent local biosynthesis of PAs within the CNS. The present investigation aimed to unequivocally answer the question of whether PAs, especially the higher ones like spermidine (SPD) and spermine (SPM), can or cannot be taken up into the brain from the bloodstream. For this purpose, a biotin-labelled analogue of spermine (B-X-SPM) was synthesized, characterized, and used to visualize its uptake into brain cells following application to acute brain slices, to the intraventricular space, or to the bloodstream. In acute brain slices there is strong uptake of B-X-SPM into protoplasmic and none in fibrous-type astrocytes. It is also taken up by neurons but to a lesser degree. Under in vivo conditions, astrocyte uptake of B-X-SPM from the brain interstitial fluid is also intense after intraventricular application. In contrast, following intracardial injection, there is no uptake from the bloodstream, indicating that the brain is completely dependent on the local synthesis of polyamines.

Uptake of Biotinylated Spermine in Astrocytes: Effect of Cx43 siRNA, HIV-Tat Protein and Polyamine Transport Inhibitor on Polyamine Uptake

Polyamines (PAs) are polycationic biomolecules containing multiple amino groups. Patients with HIV-associated neurocognitive disorder (HAND) have high concentrations of the polyamine N-acetylated spermine in their brain and cerebral spinal fluid (CSF) and have increased PA release from astrocytes. These effects are due to the exposure to HIV-Tat. In healthy adult brain, PAs are accumulated but not synthesized in astrocytes, suggesting that PAs must enter astrocytes to be N-acetylated and released. Therefore, we tested if Cx43 hemichannels (Cx43-HCs) are pathways for PA flux in control and HIV-Tat-treated astrocytes. We used biotinylated spermine (b-SPM) to examine polyamine uptake. We found that control astrocytes and those treated with siRNA-Cx43 took up b-SPM, similarly suggesting that PA uptake is via a transporter/channel other than Cx43-HCs. Surprisingly, astrocytes pretreated with both HIV-Tat and siRNA-Cx43 showed increased accumulation of b-SPM. Using a novel polyamine transport inhibitor (PTI), trimer 44NMe, we blocked b-SPM uptake, showing that PA uptake is via a PTI-sensitive transport mechanism such as organic cation transporter. Our data suggest that Cx43 HCs are not a major pathway for b-SPM uptake in the condition of normal extracellular calcium concentration but may be involved in the release of PAs to the extracellular space during viral infection.

Agmatine: multifunctional arginine metabolite and magic bullet in clinical neuroscience?

Agmatine, the decarboxylation product of arginine, was largely neglected as an important player in mammalian metabolism until the mid-1990s, when it was re-discovered as an endogenous ligand of imidazoline and α2-adrenergic receptors. Since then, a wide variety of agmatine-mediated effects have been observed, and consequently agmatine has moved from a wallflower existence into the limelight of clinical neuroscience research. Despite this quantum jump in scientific interest, the understanding of the anabolism and catabolism of this amine is still vague. The purification and biochemical characterization of natural mammalian arginine decarboxylase and agmatinase still are open issues. Nevertheless, the agmatinergic system is currently one of the most promising candidates in order to pharmacologically interfere with some major diseases of the central nervous system, which are summarized in the present review. Particularly with respect to major depression, agmatine, its derivatives, and metabolizing enzymes show great promise for the development of an improved treatment of this common disease.

The role of glia in stress: polyamines and brain disorders

This review focuses on the roles of glia and polyamines (PAs) in brain function and dysfunction, highlighting how PAs are one of the principal differences between glia and neurons. The novel role of PAs, such as putrescine, spermidine, and spermine and their precursors and derivatives, is discussed. However, PAs have not yet been a focus of much glial research. They affect many neuronal and glial receptors, channels, and transporters. They are therefore key elements in the development of many diseases and syndromes, thus forming the rationale for PA-focused and glia-focused therapy for these conditions.

Transport Reversal during Heteroexchange: A Kinetic Study

It is known that secondary transporters, which utilize transmembrane ionic gradients to drive their substrates up a concentration gradient, can reverse the uptake and instead release their substrates. Unfortunately, the Michaelis-Menten kinetic scheme, which is popular in transporter studies, does not include transporter reversal, and it completely neglects the possibility of equilibrium between the substrate concentrations on both sides of the membrane. We have developed a complex two-substrate kinetic model that includes transport reversal. This model allows us to construct analytical formulas allowing the calculation of a "heteroexchange" and "transacceleration" using standard Michaelis coefficients for respective substrates. This approach can help to understand how glial and other cells accumulate substrates without synthesis and are able to release such substrates and gliotransmitters.

Increased plasma agmatine levels in patients with schizophrenia

Agmatine is an endogenous substance, synthesized from l-arginine, and it is proposed to be a new neurotransmitter. Preclinical studies indicated that agmatine may have an important role in the pathophysiology of schizophrenia. This study was organized to investigate plasma agmatine in patients with schizophrenia and in healthy controls. Eighteen patients with schizophrenia and 19 healthy individuals constituted the subjects. Agmatine levels in the plasma were measured using the HPLC method. The S100B protein level, which is a peripheral biomarker for brain damage, was also measured using the ELISA method. While plasma levels of agmatine in patients with schizophrenia were significantly increased (p < 0.0001) compared to those of healthy individuals (control), there were no significant changes in the levels of S100B protein (p = 0.660). An ROC (receiver operating characteristic) curve analysis revealed that measuring plasma agmatine levels as a clinical diagnostic test would significantly differentiate between patients with schizophrenia and those in the control group (predictive value: 0.969; p < 0.0001). The predictive value of S100B measurements was not statistically significant (p > 0.05). A multiple regression analysis revealed that the age of the patient and the severity of the illness, as indicated by the PANSS score, significantly contributed the plasma agmatine levels in patients with schizophrenia. These results support the hypothesis that an excess agmatine release is important in the development of schizophrenia. The findings also imply that the plasma agmatine level may be a potential biomarker of schizophrenia.

Arginase and Arginine Decarboxylase - Where Do the Putative Gate Keepers of Polyamine Synthesis Reside in Rat Brain?

Polyamines are important regulators of basal cellular functions but also subserve highly specific tasks in the mammalian brain. With this respect, polyamines and the synthesizing and degrading enzymes are clearly differentially distributed in neurons versus glial cells and also in different brain areas. The synthesis of the diamine putrescine may be driven via two different pathways. In the “classical” pathway urea and carbon dioxide are removed from arginine by arginase and ornithine decarboxylase. The alternative pathway, first removing carbon dioxide by arginine decarboxlyase and then urea by agmatinase, may serve the same purpose. Furthermore, the intermediate product of the alternative pathway, agmatine, is an endogenous ligand for imidazoline receptors and may serve as a neurotransmitter. In order to evaluate and compare the expression patterns of the two gate keeper enzymes arginase and arginine decarboxylase, we generated polyclonal, monospecific antibodies against arginase-1 and arginine decarboxylase. Using these tools, we immunocytochemically screened the rat brain and compared the expression patterns of both enzymes in several brain areas on the regional, cellular and subcellular level. In contrast to other enzymes of the polyamine pathway, arginine decarboxylase and arginase are both constitutively and widely expressed in rat brain neurons. In cerebral cortex and hippocampus, principal neurons and putative interneurons were clearly labeled for both enzymes. Labeling, however, was strikingly different in these neurons with respect to the subcellular localization of the enzymes. While with antibodies against arginine decarboxylase the immunosignal was distributed throughout the cytoplasm, arginase-like immunoreactivity was preferentially localized to Golgi stacks. Given the apparent congruence of arginase and arginine decarboxylase distribution with respect to certain cell populations, it seems likely that the synthesis of agmatine rather than putrescine may be the main purpose of the alternative pathway of polyamine synthesis, while the classical pathway supplies putrescine and spermidine/spermine in these neurons.

A glutamatergic projection from the lateral hypothalamus targets VTA-projecting neurons in the lateral habenula of the rat

Homeostasis describes the fundamental biological ability of individuals to maintain stable internal conditions in a changing environment. Homeostatic reactions include internal adjustments as well as behavioral responses. In vertebrates, behavioral responses are induced by the reward system. This system originates in the ventral tegmental area (VTA) and leads to increased dopamine levels in the forebrain whenever activated. A major inhibitor of VTA activity is the lateral habenula (LHb). This epithalamic structure is able to almost completely suppress dopamine release, either directly or via the rostromedial tegmental nucleus (RMTg), when rewarding expectations are not met. A major input to the LHb arises from the lateral hypothalamic area (LHA), an important regulator of the homeostatic system. Currently, little is known about the effects of the strong hypothalamic projection on the activity of LHb neurons. In the present study, we analyze neurotransmitters and cellular targets of the LHA-LHb projection in the rat. Therefore, anterograde tracing from the LHA was combined with the visualization of neurotransmitters in the LHb. These experiments revealed a mainly glutamatergic projection, probably exerting excitatory effects on the targeted LHb cells. These cellular targets were analyzed in a second step. Anterograde tracing from the LHA in combination with retrograde tracing from the VTA/RMTg region revealed that LHb neurons projecting to the VTA/RMTg region are densely targeted by the LHA projection. Visualization of synaptophysin at these contact sites indicates that the contact sites indeed are synapses. Taken together, the present study describes a strong mainly glutamatergic projection from the LHA that targets VTA/RMTg-projecting neurons in the LHb. These findings emphasize the potential role of the LHb as direct link between homeostatic areas and reward circuitries, which may be important for the control of homeostatic behaviors.

Putative agmatinase inhibitor for hypoxic-ischemic new born brain damage

Agmatine is an endogenous brain metabolite, decarboxylated arginine, which has neuroprotective properties when injected intraperitoneally (i.p.) into rat pups following hypoxic-ischemia. A previous screen for compounds based on rat brain lysates containing agmatinase with assistance from computational chemistry, led to piperazine-1-carboxamidine as a putative agmatinase inhibitor. Herein, the neuroprotective properties of piperazine-1-carboxamidine are described both in vitro and in vivo. Organotypic entorhinal-hippocampal slices were firstly prepared from 7-day-old rat pups and exposed in vitro to atmospheric oxygen depletion for 3 h. Upon reoxygenation, the slices were treated with piperazine-1-carboxamidine or agmatine (50 μg/ml agents), or saline, and 15 h later propidium iodine was used to stain. Piperazine-1-carboxamidine or agmatine produced substantial in vitro protection compared to post-reoxygenated saline-treated controls. An in vivo model involved surgical right carotid ligation followed by exposure to hypoxic-ischemia (8 % oxygen) for 2.5 h. Piperazine-1-carboxamidine at 50 mg/kg i.p. was given 15 min post-reoxygenation and continued twice daily for 3 days. Cortical agmatine levels were elevated (+28.5 %) following piperazine-1-carboxamidine treatment with no change in arginine or its other major metabolites. Histologic staining with anti-Neun monoclonal antibody also revealed neuroprotection of CA1-3 layers of the hippocampus. Until endpoint at 22 days of age, no adverse events were observed in treated pups' body weights, rectal temperatures, or prompted ambulation. Piperazine-1-carboxamidine therefore appears to be a neuroprotective agent of a new category, agmatinase inhibitor.