Purification, crystallisation and preliminary X-ray diffraction study of ribosome inactivating protein: saporin

We report here the crysallisation and molecular replacement results on the structure determination of S-9 isoform of the ribosome inactivating protein saporin. The protein was purified to homogeneity by a simple and efficient protocol. The crystals belong to the space group I4l with a = b = 91.47 A, c = 150.66 A and contain two molecules in the asymmetric unit.

Alteration of NMDA receptor-mediated synaptic responses in CA1 area of the aged rat hippocampus: contribution of GABAergic and cholinergic deficits

Synaptic responses mediated by the N-methyl-D-aspartate receptor (NMDAr) and non-NMDAr activation were compared in CA1 hippocampal region of young (3-4 months old) and aged (25-33 months old) Sprague-Dawley rats with the use of ex vivo extracellular recordings techniques. In aged rats, the amplitude of the NMDAr-mediated field excitatory postsynaptic potentials (fEPSPs) was not altered, whereas their duration was significantly increased. In contrast, the magnitude of non-NMDAr-mediated fEPSPs was significantly smaller. The presynaptic fiber volley was not affected by age. Considering that the depression of non-NMDAr-mediated responses was previously attributed to fewer synaptic contacts between glutamatergic afferent fibers and pyramidal cells in aged animals (see Barnes et al., Hippocampus 1992;2:457-468), the absence of age-related changes in the amplitude of NMDAr-mediated fEPSPs suggests that compensatory mechanisms may occur. The contribution of gamma-aminobutyric acid (GABA) and acetylcholine to these mechanisms was addressed. The NMDAr-mediated fEPSPs were then recorded (1) in young and aged rats before and after blockade of the GABA(B) receptor-mediated inhibition by the specific antagonist CGP 55845 and (2) in young rats after a selective cholinergic denervation of the hippocampus by the immunotoxin 192 IgG-saporin. The results did not indicate statistically relevant age-related effects of CGP 55845. In contrast, the loss of the cholinergic innervation by the immunotoxin induced a significant increase in both the amplitude and duration of the NMDAr-mediated fEPSPs. Our results indicate that the functional properties of the ionotropic glutamate receptor subtypes located on CA1 pyramidal cells are differentially affected by aging and suggest that the cholinergic deficit that occurs during aging may be involved in the maintenance of robust NMDAr-mediated synaptic responses.

Pig fetal septal neurons implanted into the hippocampus of aged or cholinergic deafferented rats grow axons and form cross-species synapses in appropriate target regions

The anatomical specificity of axon growth from fetal pig septal xenografts was studied by transplanting septal cells from E30-35 pig fetuses into cholinergic deafferented (192-IgG-saporin-infused) rats or into aged rats (> 18 months). Cell suspensions (100,000 cells/microl) were injected bilaterally into the dorsal and ventral hippocampus of immunosuppressed rats (10 mg/kg/day cyclosporine A). To assess axonal growth and synapse formation, acetylcholinesterase histochemistry, an antibody to choline acetyltransferase (ChAT), and three pig-positive/rat-negative antibodies: bovine 70kD neurofilament (NF70), human low-affinity NGF receptor (hNGFr), and human synaptobrevin (hSB) were used. In rats with surviving grafts at 6 months, NF70 axonal labeling was more extensive than either ChAT or hNGFr labeling. All three markers demonstrated graft axons extending selectively through the hippocampal CA fields and the molecular layer of the dentate gyrus. Graft axons did not extend into adjacent entorhinal cortex or neocortex. The distribution of pig hSB-positive synapses correlated with AChE-positive fiber outgrowth in to the host. Electron microscopic analysis of hSB-immunostained hippocampal sections revealed pig presynaptic terminals in contact with normal rat postsynaptic structures in the CA fields and the dentate gyrus. These data demonstrate target-appropriate growth of pig cholinergic axons and the formation of cross-species synapses in the deafferented or aged rat hippocampus.

Prediction of a conserved, neutralizing epitope in ribosome-inactivating proteins

The secondary structures, side-chain solvent accessibilities, and superpositioned crystal structures of the A-chain of ricin and four other plant rRNA N-glycosidases (ribosome-inactivating proteins, RIPs) were examined. Previously, a 26-residue fragment from the A-chain of ricin was determined to bind to a neutralizing monoclonal antibody. The region in the native ricin A-chain, to which this peptide corresponds, is solvent-exposed and contains a negatively charged residue that has been hypothesized to participate in the toxin's function, namely, rRNA binding and/or enzymatic activity. This region appears to be conserved in all of the structurally defined plant RIPs examined. Moreover, other plant RIPs, whose tertiary structures are, as yet, unknown, were predicted to have an analogous, solvent-exposed region containing a conserved, negatively charged residue. By analogy, these conserved structural and functional features lead to the suggestion that this exposed region represents a logical starting point for experiments designed to locate neutralizing epitopes in these RIPs. In contrast, the tertiary structure of the analogous region in a bacteria-derived RIP (Shiga toxin) is a less solvent-exposed, truncated loop and is a structure that is not as likely to be a neutralizing epitope. Because most of the amino acid residues are not conserved within this exposed region, these RIPs are predicted to be antigenically distinct.

Iodo-QNB cortical binding and brain perfusion: effects of a cholinergic basal forebrain lesion in the rat

This study deals with the question of whether in vivo application of [125I]iodo-quinuclidinyl-benzilate (QNB) is able to demonstrate changes in cortical muscarinic receptor density induced by a cholinergic immunolesion of the rat basal forebrain cholinergic system, and whether the potential effects on IQNB distribution in vivo are also associated with effects on regional cerebral perfusion. Immunolesioned and control animals were injected with (R,S) [125]iodo-QNB and with [99mTc]-d,l-hexamethylpropyleneamine oxime (HMPAO). The cerebral distribution of both tracers was imaged using double tracer autoradiography. Impaired cholinergic transmission was paralleled by a 10-15% increase of [125I]iodo-QNB binding in the regions of cortex and hippocampus. The local cerebral blood flow remained unchanged after cholinergic lesion.

Morris water maze analysis of 192-IgG-saporin-lesioned rats and porcine cholinergic transplants to the hippocampus

Adults rats were lesioned with 192-IgG-saporin, an immunotoxin that targets cholinergic neurons in the basal forebrain expressing the low-affinity nerve growth factor receptor (p75). One month later, rats received E30-35 porcine cholinergic neurons bilaterally into the hippocampus, and were tested in the Morris water maze and the passive avoidance task 4.5-6 months after transplantation (in two experiments, rats were retested in the water maze) followed by histological and cellular analyses. The 192-IgG-saporin-lesioned animals displayed clear cognitive deficits in the Morris water maze. In all experiments the lesioned animals had spatial probe deficits on day 5 testing. A large variance was found among the transplanted animals, with individual animals exhibiting improved performance, but little overall improvement when compared to lesion-alone animals as a group. The relationships between behavioral performance and graft cholinergic factors were established by histological analyses. Grafted animals exhibited an increase in cholinergic innervation of the dentate gyrus (DG) region of the dorsal hippocampus when compared to lesion-alone animals. There was a significant correlation between the level of cholinergic innervation in the dentate gyrus and spatial navigation performance (latency and spatial probe) in the Morris water maze task. These data provide evidence of memory and spatial deficits following cholinergic denervation, and of target-specific growth of xenogeneic cholinergic neurons into the hippocampus. The lack of a clear treatment (transplant) effect in the behavioral measures leads us to believe that functional restoration of cognitive function would require cholinergic reinnervation of both the hippocampus and the neocortex in this 192-IgG-saporin animal model.

Analysis of active site residues of the antiviral protein from summer leaves from Phytolacca americana by site-directed mutagenesis

The summer leaf isoform of the pokeweed (Phytolacca americana) antiviral protein, PAP II, was produced in high yields from inclusion bodies in recombinant E. coli. On the basis of its sequence similarity with the spring leaf isoform (PAP I) and with the A chain of ricin, a three-dimensional model of the protein was constructed as an aid in the design of active site mutants. PAP II variants mutated in residues Asp 88 (D88N), Tyr 117 (Y117S), Glu 172 (E172Q), Arg 175 (R175H) and a combination of Asp 88 and Arg 175 (D88N/R175H) were produced in E. coli and assayed for their ability to inhibit protein synthesis in a rabbit reticulocyte lysate. All of these mutations had effects deleterious to the enzymatic activity of PAP II. The results were interpreted in the light of three reaction mechanisms proposed for ribosome-inactivating proteins (RIPs). We conclude that none of the proposed mechanisms is entirely consistent with the data presented here.

Host-mediated antibody-dependent cellular cytotoxicity contributes to the in vivo therapeutic efficacy of an anti-CD7-saporin immunotoxin in a severe combined immunodeficient mouse model of human T-cell acute lymphoblastic leukemia

We have investigated the anti-leukemia effect that is exerted by the murine anti-CD7 antibody HB2 in a severe combined immunodeficient (SCID) mouse model of human T-cell acute lymphoblastic leukemia (T-ALL) and determined the contribution that this antibody effect makes to the therapeutic potency of a saporin immunotoxin (IT) constructed with the same antibody. The anti-leukemia effect is not exerted through complement-mediated lysis or through direct growth-inhibitory signaling after binding of antibody to the CD7 molecule on the T-ALL cell surface but rather through antibody-dependent cellular cytotoxicity (ADCC). Thus, the in vivo depletion of SCID mice of their natural killer cells almost completely abolishes the therapeutic effect of native HB2 anti-CD7 antibody and moreover significantly reduces the in vivo therapeutic performance of the anti-CD7 HB2-SAPORIN IT. Furthermore, an IT constructed with the F(ab')2 fragment of the same anti-CD7 antibody (HB2-F(ab')2-SAPORIN), which is incapable of recruiting natural killer cells, performed significantly less well therapeutically than HB2-SAPORIN IT. There was also a significant improvement in the therapeutic performance of the HB2-F(ab')2-SAPORIN IT in SCID-HSB-2 mice when used in combination with intact HB2 antibody, presumably through restoration of an ADCC attack on the target HSB-2 cell. These combined data indicate that ADCC in the SCID mouse does contribute additively together with toxin to the in vivo therapeutic potency of the HB2-SAPORIN IT directed against this human T-ALL cell line and that this has potentially important implications for the utility of this and other related classes of immunotherapeutic in human therapy.

Effect of N-terminal deletions on the activity of pokeweed antiviral protein expressed in E. coli

Pokeweed antiviral protein (PAP) from Phytolacca americana is a highly specific N-glycosidase removing adenine residues (A4324 in 28S rRNA and A2660 in 23S rRNA) from intact ribosomes of both eukaryotes and prokaryotes. Due to the ribosome impairing activity the gene coding for mature PAP has not been expressed so far in bacteria whereas the full-length gene (coding for the mature 262 amino acids plus two signal peptides of 22 and 29 amino acids at both N- and C-termini, respectively) has been expressed in Escherichia coli. In order to determine: 1) the size of the N-terminal region of PAP which is required for toxicity to E. coli; and 2) the location of the putative enzymatic active site of PAP, 5'-terminal progressive deletion of the PAP full-length gene was carried out and the truncated forms of the gene were cloned in a vector containing a strong constitutive promoter and a consensus Shine-Dalgarno ribosome binding site. The ribosome inactivation or toxicity of the PAP is used as a phenotype characterized by the absence of E. coli colonies, while the mutation of PAP open reading frames in the small number of survived clones is used as an indicator of the toxicity to E. coli cells. Results showed that the native full-length PAP gene was highly expressed and was not toxic to E. coli cells although in vitro ribosome inactivating activity assay indicated it was active. However, all of the N-terminal truncated forms (removal of seven to 107 codons) of the PAP gene were toxic to E. coli cells and were mutated into either out of frame, early termination codon or inactive form of PAP (i.e., clone PAP delta107). Deletion of more than 123 codons restored the correct gene sequence but resulted in the loss of the antiviral and ribosome inactivating activities and by the formation of a large number of clones. These results suggest that full-length PAP (with N- and C-terminal extensions) might be an inactive form of the enzyme in vivo presumably by inclusion body formation or other unknown mechanisms and is not toxic to E. coli cells. However, it is activated by at least seven codon deletions at the N-terminus. Deletions from seven through to 107 amino acids were lethal to the cells and only mutated forms (inactive) of the gene were obtained. But deletion of more than 123 amino acids resulted in the loss of enzymatic activity and made it possible to express the correct PAP gene in E. coli. Because deletion of Tyr94 and Val95, which are involved in the binding of the target adenine base, did not abolish the activity of PAP, it is concluded that the location previously proposed for PAP enzymatic active site should be reassessed.

Reduced toxicity and broad spectrum resistance to viral and fungal infection in transgenic plants expressing pokeweed antiviral protein II

Pokeweed antiviral protein II (PAPII), a 30 kDa protein isolated from leaves of Phytolacca americana, inhibits translation by catalytically removing a specific adenine residue from the large rRNA of the 60S subunit of eukaryotic ribosomes. The protein sequence of PAPII shows only 41% identity to PAP and PAP-S, two other antiviral proteins isolated from pokeweed. We isolated a cDNA corresponding to PAPII and introduced it into tobacco plants. PAPII expressed in transgenic tobacco was correctly processed to the mature form as in pokeweed and accumulated to at least 10-fold higher levels than wild-type PAP. We had previously observed a significant decrease in transformation frequency with PAP and recovered only two transgenic lines expressing 1-2 ng per mg protein. In contrast, eight different transgenic lines expressing up to 250 ng/mg PAPII were recovered, indicating that PAPII is less toxic than PAP. Two symptomless transgenic lines expressing PAPII were resistant to tobacco mosaic virus, potato virus X and the fungal pathogen Rhizoctonia solani. The level of viral and fungal resistance observed correlated well with the amount of PAPII protein accumulated. Pathogenesis-related protein PR1 was constitutively expressed in transgenic lines expressing PAPII. Although PR1 was constitutively expressed, no increase in salicylic acid levels was detected, indicating that PAPII may elicit a salicylic acid-independent signal transduction pathway.

The deoxyribonuclease activity attributed to ribosome-inactivating proteins is due to contamination

The mode of action of ribosome-inactivating proteins (RIPs) has, for many years, been considered to be depurination of a specific adenyl residue of ribosomal RNA, resulting in inhibition of protein synthesis. Recently, this view has been challenged by the observation that many RIP preparations have significant DNase activity in addition to their N-glycosidase activity. In this study, we have investigated the putative DNase activity of two RIPs, ricin and pokeweed antiviral protein (PAP), and show that, in both cases, the DNase activity is due to the presence of contaminating nucleases. The N-glycosidase and DNase activities of PAP were separately and specifically inactivated by chemical modification and heat. Gel filtration of ricin allowed physical separation of the two activities. Furthermore, neither recombinant PAP nor recombinant ricin A-chain purified from Escherichia coli displayed DNase activity.

Sustained attention performance in rats with intracortical infusions of 192 IgG-saporin-induced cortical cholinergic deafferentation: effects of physostigmine and FG 7142

Rats with extensive lesions of cortical cholinergic afferents as a result of infusions of 192 IgG-saporin into the basal forebrain show persistent impairments in sustained attention performance (J. McGaughy, T. Kaiser, & M. Sarter, 1996). However, the administration of neither the cholinesterase inhibitor physostigmine nor the benzodiazepine receptor partial inverse agonist FG 7142 attenuated the lesion-induced impairments in performance. The present study demonstrated that less extensive cortical cholinergic deafferentation, produced by intracortical infusions of a relatively small concentration of 192 IgG-saporin, resulted in a significant impairment in sustained attention. However, the administration of neither physostigmine (0.01-0.1 mg/kg) nor FG 7142 (0.1-1.0 mg/kg) benefited the performance of the animals. Because neither compound selectively augments performance-associated increases in acetylcholine release from residual neurons, beneficial effects on cortical cholinergic deafferentation-based impairments in attention may remain limited.

Chronic elevation of amyloid precursor protein expression in the neocortex and hippocampus of rats with selective cholinergic lesions

Overexpression or aberrant processing of the beta-amyloid precursor protein (APP) and loss of cortical cholinergic function represent two hallmark pathological features of Alzheimer's disease, although it is still unclear whether these alterations take place independently or in an inter-related manner. In the present study, the possible relationships between altered APP expression and cholinergic hypofunction in the neocortex and hippocampus were addressed histologically following selective and complete (90-95%) removal of the basal forebrain cholinergic neurons by the 192 IgG-saporin immunotoxin, at a dose (5.0 microg, intraventricularly) producing profound and permanent cognitive deficits. Computer-aided densitometric analyses revealed, at 6 months post-lesion, a virtually complete loss of terminal cholinergic innervation in various neocortical and hippocampal regions (up to 80%), which correlated highly with the marked (up to 71%) increases in APP expression measured in the same territories. The present results indicate that the integrity of ascending basal forebrain cholinergic inputs to the neocortex and hippocampus may be required for the maintenance of physiological levels of APP expression in the same regions, thus providing a novel rationale for interventions aimed at restoring or enhancing cortical cholinergic neurotransmission.

Targeted delivery of DNA encoding cytotoxic proteins through high-affinity fibroblast growth factor receptors

Nonviral DNA delivery strategies for gene therapy have generally been limited by a lack of specificity and efficacy. However, ligand-mediated endocytosis can specifically deliver DNA in vitro to cells bearing the appropriate cognate receptors. Similarly, in order to circumvent problems related to efficacy, DNA must encode proteins with high intrinsic activities. We show here that the ligand basic fibroblast growth factor (FGF2) can target FGF receptor-bearing cells with DNA encoding therapeutic proteins. Delivery of genes encoding saporin, a highly potent ribosomal inactivating protein, or the conditionally cytotoxic herpes simplex virus thymidine kinase, a protein that can kill cells by activating the prodrug ganciclovir, is demonstrated. The saporin gene was codon optimized for mammalian expression and demonstrated to express functional protein in a cell-free assay. FGF2-mediated delivery of saporin DNA or thymidine kinase DNA followed by ganciclovir treatment resulted in a 60 and 75% decrease in cell number, respectively. Specificity of gene delivery was demonstrated in competition assays with free FGF2 or with recombinant soluble FGF receptor. Alternatively, when histone H1, a ligand that binds to cell surface heparan sulfate proteoglycans ("low-affinity" FGF receptors), was used to deliver DNA encoding thymidine kinase, no ganciclovir sensitivity was observed. These findings establish the feasibility of using ligands such as FGF2 to specifically deliver genes encoding molecular chemotherapeutic agents to cells.

Septocingulate and septohippocampal cholinergic pathways: involvement in working/episodic memory

The contribution of the septohippocampal cholinergic pathway to performance of a working/episodic memory task was compared to that of the septocingulate cholinergic path. The septocingulate and septohippocampal cholinergic pathways were selectively destroyed in male Sprague-Dawley rats using site-specific injections of the anti-neuronal immunotoxin 192-IgG saporin into either the hippocampus or the cingulate cortex. 192-IgG-saporin selectively destroys cholinergic neurons and terminals that express the p75 neurotrophin receptor. Following extensive pre-operative training, working memory was assessed using a delayed nonmatch to sample eight arm radial maze task, with delays of 1, 4 and 8 h. The group with lesions of the septohippocampal cholinergic pathway displayed performance deficits on this task which were not related to length of delay. In contrast, the group with lesions of the septocingulate cholinergic pathway did display delay-dependent deficits which were observed at the 4- and 8-h delays, but not at the 1-h delay. These data suggest that the septocingulate cholinergic pathway is critically involved in working/episodic memory but that the septohippocampal cholinergic pathway is either not contributing to working/episodic memory per se or it is involved only at shorter delays.

The behavioral functions of the cholinergic basal forebrain: lessons from 192 IgG-saporin

Until recently our understanding of the functional neuroanatomy of the cholinergic basal forebrain (CBF) has been hindered by the lack of a lesioning technique that is truly selective. The development of the immunotoxin 192 IgG-saporin (192-sap) has greatly improved our ability to create specific lesions of the CBF. Rats with such lesions have been studied in a wide variety of behavioral paradigms of learning, memory, and attention. Complete or near-complete destruction of the CBF results in deficits in a variety of behavior paradigms including passive avoidance, spatial tasks (water and radial mazes), delayed matching to position/sample, and attentional tasks. However, interpretation of many experiments is hampered by incomplete lesions and/or concomitant damage to cerebellar Purkinje neurons. Future studies will need to address these issues. Recent development of a similar immunotoxin that is effective in primates should permit more sophisticated behavioral analysis of CBF function. Additionally, immunotoxins selective for other types of neurons, such as the noradrenergic selective anti-DBH-saporin, will permit analysis of the behavioral functions of other diffusely projecting systems and how these other systems may interact with the CBF.

Glucose metabolism in cholinoceptive cortical rat brain regions after basal forebrain cholinergic lesion

To address the question whether the changes in cortical glucose metabolism observed in patients with Alzheimer's disease are interrelated with, or consequences of, basal forebrain cholinergic cell loss, an experimental approach was employed to produce cortical cholinergic dysfunction in rat brain by administration of the cholinergic immunotoxin 192IgG-saporin. [14C]D-glucose utilization in brain homogenates, D-glucose-displaceable [3H]cytochalasin B binding to glucose transporters (GLUT). Northern and Western analyses, as well as in vivo [14C]2-deoxyglucose autoradiography were used to quantify the regional glucose metabolism. Basal forebrain cholinergic lesion resulted in transient increases in glucose transporter binding in cortical regions displaying reduced acetylcholinesterase activity, already detectable seven days after lesion with peak values around 30 days post lesion. Western analysis revealed that the changes in total glucose transporter binding are mainly due to changes in the GLUT3 subtype only, while the levels of GLUT1 and GLUT3 mRNA (Northern analysis) were not affected by cholinergic lesion. Both immunocytochemistry and in situ hybridization demonstrated preferential localizations of GLUT1 on brain capillaries and GLUT3 on neurons, respectively. A lesion-induced transient decrease in [14C]D-glucose utilization seven days post lesion was detected in the lesion site, whereas cholinoceptive cortical regions were not affected. In vivo [14C]deoxyglucose uptake was transiently increased in cholinoceptive cortical regions and in the lesion site being highest between three to seven days after lesion. The cholinergic lesion-induced transient up-regulation of cortical glucose transporters and deoxyglucose uptake reflects an increased glucose demand in regions depleted by acetylcholine suggesting functional links between cortical cholinergic activity and glucose metabolism in cholinoceptive target regions.

The biologic effects of growth factor-toxin conjugates in models of vascular injury depend on dose, mode of delivery, and animal species

Toxin-conjugates, complexes designed from the fusion of tissue toxins and pathology-specific ligands, offer the potential for targeted cytotoxic therapy. Some have postulated that the recurrent failure of these conjugates to exhibit benefit in animal models of vascular injury arose because the timing and frequency of conjugate delivery were insufficient to meet the demands of the arterial wall. Previous data suggest that increasingly frequent dosing would lead to superior inhibition of intimal hyperplasia. We now report on the biological effects of the controlled release of a recombinant conjugate of basic fibroblast growth factor (bFGF) and the plant toxin saporin (SAP), bFGF-SAP. Alginate/heparin-Sepharose microspheres and films were designed as drug carriers to control release the bFGF-SAP conjugate or bFGF alone in small doses. When bFGF-SAP-incorporated microspheres or films were implanted adjacent to balloon angioplastied porcine carotid arteries, the controlled release of bFGF-SAP over the four-week study stimulated rather than inhibited hyperplasia. When these same devices were used in cell culture, unexpected findings were produced. bFGF-SAP reduced in vitro bovine vascular smooth muscle cell growth at high concentrations (1-10 microgram/mL) but increased smooth muscle cell growth at lower concentrations (up to 1 microgram/mL). Microsphere controlled-released bFGF-SAP ( approximately 60 ng/mL over 4 days) stimulated the growth of smooth muscle cells more than any of the tested bolus applications of the conjugate. These data provide cause to reconsider our acceptance of controlled release technology as the answer to all forms of drug delivery problems, and to apply more rigorous means of matching the kinetics of drug delivery to the kinetics of the vascular response to injury.

Immunolesion of the cholinergic basal forebrain: effects on functional properties of hippocampal and septal neurons

Deficits in cholinergic function have been documented in a variety of brain disorders including Alzheimer's Disease and, to a lesser extent, in normal ageing. In the present article, we have reviewed our recent findings on the effects of the loss of basal forebrain cholinergic neurons on the functional properties of the septohippocampal pathway. In vivo and ex vivo investigations were performed in rats following basal forebrain cholinergic lesion with the specific immunotoxin 192 IgG-saporin. Our results suggest a significant contribution of cholinergic neurons in the rhythmically bursting activity recorded within the medial septum. In addition, they give evidence that acetylcholine may tonically decrease the glutamatergic synaptic responses in the hippocampus whereas the GABAergic mediated inhibitory potentials are not affected. The possible contribution of these cholinergic mechanisms in the age-related functional alterations of the septohippocampal activity is discussed.

Differential changes of nicotinic receptors in the rat brain following ibotenic acid and 192-IgG saporin lesions of the nucleus basalis magnocellularis

The basal forebrain cholinergic neurons are implicated in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD). The nicotinic acetylcholine receptors (nAChRs) have been found to be significantly afflicted in AD. To study the underlying mechanisms for dysfunction of the basal forebrain cholinergic neurons development of suitable animal models is warranted. In this study we investigated the effects of bilateral lesions of the nucleus basalis magnocellularis on nAChRs in the rat brain using the cholinergic system selective immunotoxin 192-IgG saporin and non-selective excitotoxin ibotenic acid. Changes in nAChRs were measured by 3H-cytisine and 3H-epibatidine, two ligands with different selectivity for nAChRs subtypes. In the parietal cortex of ibotenic acid lesioned rates, the choline acetyltransferase activity (ChAT) was decreased by 24% while no changes were detected in the frontal cortex or hippocampus. Similarly, a 40% decrease was observed in the number of nAChRs labelled by 3H-cytisine, but not by 3H-epibatidine, in the parietal cortex, while no changes were found in the frontal cortex or hippocampus. Although the 192-IgG saporin induced lesions reduced the ChAT activity in the frontal cortex, parietal cortex and hippocampus by 77, 50 and 21%, respectively, no changes were observed in the number of nAChRs as studied by 3H-cytisine or 3H-epibatidine. The results indicate a difference in vulnerability of the cortical nAChR subtypes to experimental lesions of the nucleus basalis magnocellularis. The findings in this study suggest that a major portion of the nAChRs might be located on non-cholinergic neurons in the brain.

Effects of cholinergic depletion on evoked activity in the cortex of young and aged rats

Degeneration of cholinergic neurons in the basal forebrain is a neural marker of Alzheimer's disease and is associated with perceptual and cognitive deficits. An idea that has attracted scientific scutiny is that aging makes the brain more susceptible to neurodegenerative diseases such as Alzheimer's. The purpose of this study was to compare the effects of the loss of cholinergic input from nucleus basalis of Meynert on evoked activity in the posteromedial barrel subfield of the somatosensory cortex in young (2-2.5 months) and aged (28-30 months) male Fisher hybrid rats. The mean firing rate and receptive fields of single neurons in the posteromedial barrel subfield of the somatosensory cortex were examined after selective lesions of cholinergic neurons in the nucleus basalis of Meynert with an immunotoxin. IgG 192-saporin. Functional properties of single neurons in young animals were affected much more significantly by cholinergic depletion than those in aged animals. In cholinergic-depleted young animals, the mean firing rate of evoked activity and receptive field of posteromedial barrel subfield neurons were significantly decreased. Cholinergic depletion caused a 14% decrease in evoked activity and a 33% increase in receptive field size in young animals. The mean firing rate and receptive field of single neurons were not affected by cholinergic depletion in aged animals. It is concluded that functional properties of cortical sensory neurons in young animals are more vulnerable to cholinergic depletion than are those of aged animals and that cholinergic depletion does not further impact the properties of neurons exposed to the processes of aging.

A recombinant ribosome-inactivating protein from the plant Phytolacca dioica L. produced from a synthetic gene

Phytolacca dioica L. leaves produce at least two type-I ribosome-inactivating proteins. Each polypeptide chain is subjected to different post-translational modifications giving rise to PD-L1 and PD-L2, and PD-L3 and PD-L4, each polypeptide pair having the same primary structure. With the aim of exploiting the cytotoxic properties of these proteins as potential biological phytodrugs, a gene encoding PD-L4 was designed based on criteria expected to maximize the translation efficiency in tomato. The gene was constructed from 18 oligonucleotides and preliminarily expressed in Escherichia coli, using the T7 promoter system. The protein produced was insoluble and accumulated in inclusion bodies to about 300 mg/l of culture. Ribosome-inactivating activity was generated by controlled oxidation of the reduced and denatured protein. The recombinant protein was indistinguishable from natural PD-L4 as isolated from leaves of Phytolacca dioica, in both catalytic activity and primary structure.

192 IgG-saporin abolishes p75 neurotrophin receptor immunoreactivity in rat SCN

The rat suprachiasmatic (SCN) contains a dense plexus of low-affinity p75 neurotrophin receptor (p75NTR)-immunoreactivity. In some SCN neurons, p75NTR is co-localized with vasoactive intestinal peptide (VIP). The present study examines the effect of third ventricle administration of 192 IgG-saporin immunotoxin on p75NTR and VIP immunoreactivity in the rat SCN. The 192 IgG-saporin immunotoxin abolished p75NTR immunoreactivity in the SCN. VIP immunoreactivity in the SCN of saporin-lesioned animals was not significantly different from that of control animals. Immunolesions of the p75NTR-ir cell population in the SCN may prove useful in clarifying the role of p75NTR in circadian timing.

Responses in the aged rat brain after total immunolesion

In the present study, we compare the effects of cholinergic deafferentation of the hippocampus, cortex, and olfactory bulb of young and aged rats on nerve growth factor (NGF) protein levels in these areas. We also describe glial responses to intraventricular injections of the immunotoxin, 192 IgG-saporin in the aged. Choline acetyltransferase (ChAT) activity was dramatically decreased in the basal forebrain and target areas of the cholinergic basal forebrain neurons (CBFNs) in the young immunolesioned rats and to a lesser extent in their aged counterparts. After total immunolesion, NGF protein levels significantly increased in the hippocampus, cortex, and olfactory bulb of the young rats but not of the aged rats, except for small increases in the olfactory bulb after two weeks. After immunolesion NGF protein levels in the basal forebrain increased in young rats and less so in the aged rats. The total immunolesions had no effects on NGF and BDNF mRNA levels in the hippocampus and cortex. Two weeks after injection of the immunotoxin, the profiles of AChE- and p75NTR-positive cells significantly decreased in medial septum, vertical and horizontal limbs of diagonal band and nucleus basalis of Meynert. There was also an increase in microglia while but not astrocytes in the subnuclei of basal forebrain. In conclusion, 192 IgG-saporin was effective in producing cholinergic lesions in both young and aged rat brains, the lesion-induced NGF response was partially extinguished in the aged rat brains and immunolesions induced a microglial response in aged brain.

Differential response of human melanoma and Ehrlich ascites cells in vitro to the ribosome-inactivating protein luffin

The cytotoxicity and inhibitory effect on proliferation of the type 1 ribosome-inactivating protein luffin purified from the seeds of Luffa aegyptiaca were investigated both in human metastatic melanoma cells and in murine Ehrlich ascites tumour cells. Results indicate that luffin from the seeds of Luffa aegyptiaca is cytotoxic to the cell lines tested, with approximately 10 times greater potency in Ehrlich cells. Luffin was found to induce an increase in cytosolic oligonucleosome-bound DNA in both melanoma and Ehrlich ascites tumour cells, the level of DNA fragmentation in the former cell line being higher than in the latter. Experiments with melanoma cells indicate that an increase in cytosolic nucleosomes could be supportive of apoptosis as the type of cell death induced by luffin.