Ben Avi Weissman

An Autoinhibitory Control Element Defines Calcium-regulated Isoforms of Nitric Oxide Synthase

Nitric oxide synthases (NOSs) are classified functionally, based on whether calmodulin binding is Ca2+-dependent (cNOS) or Ca2+-independent (iNOS). This key dichotomy has not been defined at the molecular level. Here we show that cNOS isoforms contain a unique polypeptide insert in their FMN binding domains which is not shared with iNOS or other related flavoproteins. Previously identified autoinhibitory domains in calmodulin-regulated enzymes raise the possibility that the polypeptide insert is the autoinhibitory domain of cNOSs. Consistent with this possibility, three-dimensional molecular modeling suggested that the insert originates from a site immediately adjacent to the calmodulin binding sequence. Synthetic peptides derived from the 45-amino acid insert of endothelial NOS were found to potently inhibit binding of calmodulin and activation of cNOS isoforms. This inhibition was associated with peptide binding to NOS, rather than free calmodulin, and inhibition could be reversed by increasing calmodulin concentration. In contrast, insert-derived peptides did not interfere with the arginine site of cNOS, as assessed from [3H]N G-nitro-l-arginine binding, nor did they potently effect iNOS activity. Limited proteolysis studies showed that calmodulin’s ability to gate electron flow through cNOSs is associated with displacement of the insert polypeptide; this is the first specific calmodulin-induced change in NOS conformation to be identified. Together, our findings strongly suggest that the insert is an autoinhibitory control element, docking with a site on cNOSs which impedes calmodulin binding and enzymatic activation. The autoinhibitory control element molecularly defines cNOSs and offers a unique target for developing novel NOS activators and inhibitors.

NG-nitro-L-arginine enhances neuronal death following transient forebrain ischemia in gerbils.

Experiments were performed with Mongolian gerbils to study the effect of the specific nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (L-NNA) on ischemic brain damage induced by 5 min bilateral carotid occlusion. A single i.p. injection of L-NNA did not result in any neuronal loss in the central nervous system. In animals undergoing ischemia, a selective destruction of hippocampal CA1 cells was observed whereas pretreatment with 50 mg/kg L-NNA 4 h before administration of ischemia produced significantly more extensive cell damage in the hippocampus and other brain regions. These findings demonstrate that in this model inhibition of nitric oxide generation augments ischemia-induced neuronal cell injury in the brain.

Caramiphen and Scopolamine Prevent Soman-Induced Brain Damage and Cognitive Dysfunction

Exposure to soman, a toxic organophosphate nerve agent, causes severe adverse effects and long term changes in the peripheral and central nervous systems. The goal of this study was to evaluate the ability of prophylactic treatments to block the deleterious effects associated with soman poisoning. scopolamine, a classical anticholinergic agent, or caramiphen, an anticonvulsant anticholinergic drug with anti-glutamatergic properties, in conjunction with pyridostigmine, a reversible cholinesterase inhibitor, were administered prior to sbman (1 LD50). Both caramiphen and scopolamine dramatically attenuated the process of cell death as assessed by the binding of [3H]RoS-4864 to peripheral benzodiazepine receptors (omega3 sites) on microglia and astrocytes. In addition, caramiphen but not scopolamine, blocked the soman-evoked down-regulation of [3H]AMPA binding to forebrain membrane preparations. Moreover, cognitive tests utilizing the Morris water maze, examining learning and memory processes as well as reversal learning, demonstrated that caramiphen abolished the effects of soman intoxication on learning as early as the first trial day, while scopolamine exerted its effect commencing at the second day of training. Whereas the former drug completely prevented memory deficits, the latter exhibited partial protection. Both agents equally blocked the impairment of reversal learning. In addition, there is a significant correlation between behavioral parameters and [3H]RoS-4864 binding to forebrain membrane preparations of rats, which participated in these tests (r(21) = 0.66, P < 0.001; r(21) = 0.66, P < 0.001, -0.62, P < 0.002). These results demonstrate the beneficial use of drugs exhibiting both anti-cholinergic and anti-glutamatergic properties for the protection against changes in cognitive parameters caused by nerve agent poisoning. Moreover, agents such as caramiphen may eliminate the need for multiple drug therapy in organophosphate intoxications.

Therapy against organophosphate poisoning: The importance of anticholinergic drugs with antiglutamatergic properties

Potent cholinesterase inhibitors (e.g., soman, sarin), induce a wide range of deleterious effects including convulsions, behavioral impairments and ultimately, death. Due to the likelihood of various scenarios of military or terrorist attacks by these and other chemical weapons, research has to be aimed at finding optimal therapies. Early accumulation of acetylcholine in synaptic clefts was suggested to trigger an array of toxic events including an excessive release of glutamate, culminating in the activation of its receptors. Stimulation of the N-Methyl-D-Aspartate (NMDA) subtype of these receptors was associated with the neuronal injury that initiates organophosphate-induced brain damage. The notion of a stepwise mechanism yielded treatments based on a combination of an immediate administration of enzyme reactivators and anticholinergic drugs. This strategy dramatically increased survival rates but did not abolish convulsions and failed to prevent the ensuing cognitive dysfunction. Efforts to improve this paradigm by adding anticonvulsants or antiglutamatergic drugs with anti-epileptic characteristics produced dubious results. Under these conditions, benactyzine and caramiphen, agents with anticholinergic and antiglutamatergic properties, provided improved protection when introduced as adjunct agents to oximes, reversible cholinesterase inhibitors and/or specific antimuscarinic drugs such as atropine. In contrast, the specific antimuscarinic drug scopolamine failed to block soman-induced changes in glutamatergic and behavioral parameters even when given prophylactically. These findings along with a large number of additional reports led towards the conclusion that the therapeutic advantage of drugs such as benactyzine and caramiphen could derive from their ability to modulate central cholinergic and glutamate neurotransmission.

Measurement of NO and NO Synthase

Nitric oxide (NO) is a key biosignaling molecule produced in both peripheral tissues and the central nervous system by a family of enzymes known as nitric oxide synthases (NOSs). NOSs convert L‐arginine to stoichiometric quantities of NO and L‐citrulline using molecular oxygen and NADPH as cofactors. Techniques for measurement of NO and NOS activity are essential to demonstrate the role of NO and NO‐derived species in biological systems. This unit describes two methods for detection of NO: a direct method employing chemiluminescent detection and one based on quantification of the stable oxidation products with detection using the Greiss reagent. Additionally, NOS activity can be quantified by measuring the conversion of radiolabeled L‐arginine to radiolabeled L‐citrulline.

Peripheral benzodiazepine receptors: on mice and human brain imaging

There are numerous methods designed to monitor brain neuropathologies resulting from a wide arsenal of insults. Regardless of the cause of neuronal death, reactive glial cells always appear at and around the site of degeneration. These cells are distinguished by the exceptional abundance of peripheral benzodiazepine receptors, particularly compared with surrounding neurons. Measuring the binding of specific ligands to these peripheral benzodiazepine receptors offers a unique indirect marker for reliable damage assessment in the CNS and a faithful indicator for the accompanying cognitive deficits.

Activation and inactivation of neuronal nitric oxide synthase: characterization of Ca2+-dependent [125I]Calmodulin binding

Constitutive isoforms of nitric oxide synthase (NOS) are activated by transient binding of Ca(2+)/Calmodulin. Here, we characterize the binding of Calmodulin to purified neuronal NOS (nNOS). [125I]Calmodulin bound to a single class of non-interacting and high affinity sites on nNOS. [125I]Calmodulin binding achieved rapid saturation, was linear with nNOS concentration, and exhibited a strict dependence on [Ca(2+)]. Neither affinity nor extent of [125I]Calmodulin binding was affected by L-arginine, NADPH or Tetrahydrobiopterin. Native Calmodulin and engineered Calmodulin homologs [i.e., duplicated N-terminal (CaMNN)] potently displaced [125I]Calmodulin. CaMNN supported nNOS catalysis, but required approximately five-fold more Ca(2+) for comparable activity with native Calmodulin. Taken with results from kinetic analyses of [125I]Calmodulin association and dissociation, our findings suggest four sequential steps in activation of nNOS by Calmodulin: (1) Ca(2+) binds to Calmodulins C-lobe, (2) the C-lobe of Calmodulin binds NOS, (3) Ca(2+) binds to the N-lobe of Calmodulin, and (4) the N-lobe binds to nNOS. Activation of nNOS only occurs after completion of step (4), with the displacement of nNOSs autoinhibitory insert. Upon intracellular Ca(2+) sequestration, deactivation of nNOS would proceed in reverse order.

Maximal electroshock increases the density of [3H]Ro 5-4864 binding to mouse cerebral cortex

The effects of chemically and electrically-induced convulsions on the binding of [3H]Ro 5-4864 to peripheral benzodiazepine receptors (PBR) was studied in both peripheral tissues and the central nervous system (CNS). Acute, maximal electroshock (MES) increased the density of PBR in mouse cerebral cortex as evidenced by a 30% increase in the Bmax of this archetypic ligand. These values returned to control levels by 60 minutes after MES treatment. In contrast, thirty and sixty minutes after convulsions induced by Ro 5-4864, strychnine, or pentylenetetrazol, neither the Bmax nor Kd of [3H]Ro 5-4864 binding to mouse cerebral cortical membranes was altered. The increase in [3H]Ro 5-4864 binding to cortex observed 30 minutes after MES was blocked by anticonvulsant doses of phenobarbital, phenytoin and clonazepam. No changes in the characteristics of [3H]Ro 5-4864 binding was observed in cerebellar or hippocampal membranes 30 minutes following acute MES. Further, after long-term MES administration (1 treatment/day, 5 days), no change in PBR density could be detected 30 minutes after the last MES. Finally, while no change in PBR density was noted in the kidneys 30 minutes after the MES, a significant increase in PBR density was seen in the cardiac ventricles. These results demonstrate a selective modulation of PBR density by MES, suggesting that the PBR could be involved in either the generation of seizures or in postictal compensatory processes.

Age-related changes in the cholinergic components within the central nervous system II. Working memory impairment and its relation to hippocampal muscarinic receptors

Cognitive performance in aging Wistar rats was monitored using the radial arm maze and the latter was correlated with the density of muscarinic receptors in the CNS, using quantitative in vitro receptor autoradiography. Significant working memory deficits were observed in 12, 17 and 24-month-old rats as compared to 3-month-old animals. In addition, the number of the muscarinic receptors declined significantly with age (from 27 to 42% depending on the brain region sampled) utilising [3H]QNB and [3H]PZ receptor binding assays. The above trend became evident already at the age of 12 months. The present findings support the association of central cholinergic activity with memory processes.

Dose-dependent effect of nitric oxide synthase inhibition following transient forebrain ischemia in gerbils.

The extensive research concerning the interaction between nitric oxide (NO) and ischemic brain tissue has yielded contradictory results. The present study was designed to explore the effect of gradual inhibition of NO production on brain ischemia. Gerbils were administered (i.p.) either saline (control-ischemia), or 5, 10, 25 or 50 mg/kg of NG-nitro-L-arginine (NARG), a specific inhibitor of NO synthase (NOS), and 4 h later were subjected to 5 min of forebrain ischemia. A group receiving 50 mg/kg NARG with sham operation served as a second control (control-NARG) group. Body weights and spontaneous activity were monitored daily until day 6, when the gerbils were sacrificed and their brains processed for histologic-morphometric evaluation. All ischemia groups displayed significant decreases in body weights starting on day 1, as compared to control-NARG (non-ischemic) gerbils. At 24 h post-ischemia spontaneous activity was increased in all ischemia groups in a dose-dependent manner, reaching a peak at 25 mg/kg. Typical ischemia-induced neuronal cell degeneration was observed at the hippocampal CA1 layer in control-ischemia and in each of the dose-groups of 10 mg/kg NARG and above. The 5 mg/kg group displayed damage which was not different from control-NARG, and was milder (P < 0.01) than control-ischemia gerbils and each of the other dose-groups. It is suggested that during ischemia, NO activates a series of processes which are beneficial to brain tissue, whereas an excess amount of NO causes neurotoxic effects.(ABSTRACT TRUNCATED AT 250 WORDS)