Effects of intraventricular transplantation of NGF-secreting cells on cholinergic basal forebrain neurons after partial immunolesion

Intracerebral transplantation of erythropoietin-producing fibroblasts facilitates neurogenesis and functional recovery in an ischemic stroke model

INTRODUCTION

Erythropoietin (EPO) can enhance neurogenesis and fibroblasts can secrete growth factors; together, they may benefit ischemic stroke. We transplanted EPO-producing fibroblasts into the rodent infarcted brain to test their effect on neurogenesis and functional recovery.

METHODS

A total of 10⁶ cells of EPO-producing NIH/3T3 fibroblasts (EPO/EGFP/3T3) or enhanced green fluorescence protein (EGFP)-expressing fibroblasts (EGFP/3T3) were stereotaxically injected into the infarcted striatum of adult rats that received transient middle cerebral artery occlusion (MCAO) surgery 1 day poststroke. On day 14 after MCAO, the animals were euthanized for the evaluation of neurogenesis via immunohistochemistry and of the expression of growth factors using enzyme-linked immunosorbent assay. The infarct volume was analyzed using magnetic resonance imaging and the neurological behavior was assessed using the neurological severity scoring performed within 14 days after MCAO.

RESULTS

The MCAO rats with EPO/EGFP/3T3 treatment showed high EPO expression in the infarcted brain for at least 1 week. The concentration of brain-derived neurotrophic factor was higher in both hemispheres of MCAO rats with either EGFP/3T3 or EPO/EGFP/3T3 treatment at 14 days poststroke compared with untreated MCAO rats. The number of Ki-67-, nestin-, or doublecortin-immunoreactive cells in bilateral subventricular zones was higher in EPO/EGFP/3T3-treated MCAO rats than it was in untreated MCAO control animals, indicating the enhancement of neurogenesis after EPO/EGFP/3T3 treatment. Notably, post-MCAO EPO/EGFP/3T3 treatment significantly reduced infarct size and improved functional recovery.

CONCLUSION

The intracerebral transplantation of EPO-producing fibroblasts benefited an ischemic stroke model probably via the enhancement of neurogenesis.

Low-Affinity Neurotrophin Receptor p75 Promotes the Transduction of Targeted Lentiviral Vectors to Cholinergic Neurons of Rat Basal Forebrain

Basal forebrain cholinergic neurons (BFCNs) are one of the most affected neuronal types in Alzheimer's disease (AD), with their extensive loss documented at late stages of the pathology. While discriminatory provision of neuroprotective agents and trophic factors to these cells is thought to be of substantial therapeutic potential, the intricate topography and structure of the forebrain cholinergic system imposes a major challenge. To overcome this, we took advantage of the physiological enrichment of BFCNs with a low-affinity p75 neurotrophin receptor (p75NTR) for their targeting by lentiviral vectors within the intact brain of adult rat. Herein, a method is described that affords selective and effective transduction of BFCNs with a green fluorescence protein (GFP) reporter, which combines streptavidin-biotin technology with anti-p75NTR antibody-coated lentiviral vectors. Specific GFP expression in cholinergic neurons was attained in the medial septum and nuclei of the diagonal band Broca after a single intraventricular administration of such targeted vectors. Bioelectrical activity of GFP-labeled neurons was proven to be unchanged. Thus, proof of principle is obtained for the utility of the low-affinity p75NTR for targeted transduction of vectors to BFCNs in vivo.

The significance of the cholinergic system in the brain during aging and in Alzheimer's disease

Acetylcholine is widely distributed in the nervous system and has been implicated to play a critical role in cerebral cortical development, cortical activity, controlling cerebral blood flow and sleep-wake cycle as well as in modulating cognitive performances and learning and memory processes. Cholinergic neurons of the basal forebrain complex have been described to undergo moderate degenerative changes during aging, resulting in cholinergic hypofunction that has been related to the progressing memory deficits with aging. Basal forebrain cholinergic cell loss is also a consistent feature of Alzheimer's disease, which has been suggested to cause, at least partly, the cognitive deficits observed, and has led to the formulation of the cholinergic hypotheses of geriatric memory dysfunction. Impaired cortical cholinergic neurotransmission may also contribute to beta-amyloid plaque pathology and increase phosphorylation of tau protein the main component of neurofibrillar tangles in Alzheimer's disease. Understanding the molecular mechanisms underlying the interrelationship between cortical cholinergic dysfunction, beta-amyloid formation and deposition, and tau pathology in Alzheimer's disease, would allow to derive potential treatment strategies to pharmacologically intervene in the disease-causing signaling cascade.

Evaluation of cholinergic markers in Alzheimer's disease and in a model of cholinergic deficit

Cognitive deficits in neuropsychiatric disorders, such as Alzheimer's disease (AD), have been closely related to cholinergic deficits. We have compared different markers of cholinergic function to assess the best biomarker of cognitive deficits associated to cholinergic hypoactivity. In post-mortem frontal cortex from AD patients, acetylcholine (ACh) levels, cholinacetyltransferase (ChAT) and acetylcholinesterase (AChE) activity were all reduced compared to controls. Both ChAT and AChE activity showed a significant correlation with cognitive deficits. In the frontal cortex of rats with a selective cholinergic lesion, all cholinergic parameters measured (ACh levels, ChAT and AChE activities, "in vitro" and "in vivo" basal ACh release) were significantly reduced. AChE activity was associated to ChAT activity, and even more, to "in vivo" and "in vitro" basal ACh release. Quantification of AChE activity is performed by an easy and cheap method and therefore, these results suggest that determination of AChE activity may be used as an effective first step method to evaluate cholinergic deficits.

Hebb-Williams performance and scopolamine challenge in rats with partial immunotoxic hippocampal cholinergic deafferentation

Recent studies suggested that the cholinergic innervation of the hippocampus is not crucial for spatial learning, but it might be important for other forms of learning. This study assessed the effects of partial immunotoxic cholinergic lesions in the medial septum and concurrent scopolamine challenge in a complex learning task, the Hebb-Williams maze. Long-Evans rats were given intraseptal injections of 192 IgG-saporin (SAPO). Rats injected with phosphate-buffered saline (PBS) served as controls. Starting 25 days after surgery, behavioural performance was assessed in the Hebb-Williams maze test without prior or after injection of scopolamine (0.17 or 0.5 mg/kg, i.p.). In SAPO rats, histochemical analysis showed a 40-45% decrease in the density of hippocampal AChE staining. The number of ChAT-positive cell bodies in the medial septum was also significantly decreased (-56%) and there was a non-significant reduction of the number of parvalbumine-positive neurons. The behavioural results demonstrated that the lesions induced small but significant learning deficits. At 0.17 mg/kg, scopolamine produced more impairments in SAPO rats than in PBS-injected rats, suggesting an additive effect between the partial lesion and the drug. These observations indicate that the Hebb-Williams test may be more sensitive to alterations of septohippocampal cholinergic function, than radial- or water-maze tasks. They also show that subtle learning deficits can be detected after partial lesions of the cholinergic septohippocampal pathways. Finally, the data from the scopolamine challenge are in keeping with clinical results showing higher sensitivity to muscarinic blockade in aged subjects in whom weaker cholinergic functions can be presumed.

Differential changes in rat cholinergic parameters subsequent to immunotoxic lesion of the basal forebrain nuclei

The degree of lesion produced by 192 IgG-saporin relative to controls was compared using three independent methods. Microdialyzed acetylcholine (ACh), choline acetyltransferase (ChAT) activity, and the rate of ACh synthesis were compared in the frontal cortex and hippocampus. Microdialysis of rats was performed 1 and 15 weeks post-lesion. In week 16, the rats were sacrificed after an injection of deuterated choline (Ch) for determination of the rate of ACh synthesis. ChAT activity was determined at the same timepoints in a separate set of rats. At 1 week, ChAT activity and microdialyzed ACh showed similar degrees of depletion. At 15 weeks, microdialyzed ACh was significantly lower than the synthesis rate in cortex, but not in hippocampus. A small increase in ChAT activity between 1 and 15 weeks was found in the cortex, but not hippocampus. In the hippocampus, however, the rate of ACh synthesis was significantly greater than ChAT activity. This was true for two doses of immunotoxin; the greater compensation occurring with the lesser lesion. Microdialyzed ACh levels were not different from the other measures in hippocampus. Residual cholinergic terminals in the hippocampus, but not frontal cortex, compensate for a selective cholinergic lesion by increasing the rate of synthesis and may thereby alleviate hippocampus-dependent behavioral deficits.

Transplants of NGF-secreting fibroblasts restore stimulus-evoked activity in barrel cortex of basal-forebrain-lesioned rats

Cholinergic nuclei in the basal forebrain supply the cerebral cortex with acetylcholine (ACh). Depletion of cholinergic fibers following basal forebrain lesion results in reduced stimulus-evoked functional activity in rat barrel cortex in response to whisker stimulation. We showed previously that exogenous delivery of nerve growth factor (NGF) to the lateral ventricle restores reduced functional activity toward normal despite persistent reductions in cortical cholinergic activity. Gene transfer of therapeutic peptides using genetically engineered cells allows for localized and biological delivery of compounds to the CNS, circumventing systemic administration or repetitive invasive surgery. In this study, we grafted genetically engineered fibroblasts that secrete NGF (NGF+) into three CNS loci of rats with unilateral basal forebrain lesions, along with control fibroblasts (NGF-) that did not secrete NGF. Only NGF+ fibroblasts grafted into ACh-depleted somatosensory cortex resulted in improvement of functional activity following cholinergic depletion. NGF+ fibroblast transplants into the lateral ventricle or basal forebrain did not improve functional activity nor did NGF- fibroblasts in any site. Similar to our previous experiments using intraventricular NGF injections, despite improvements in functional activity, the affected barrel cortex remained depleted of acetylcholinesterase-stained fibers following insertion of NGF+ fibroblasts. These data support the idea that NGF can act directly on the cerebral cortex following reductions in cholinergic innervation. The mechanism of NGF action is elusive, however, since the presence of its high-affinity receptor, trkA, in the cerebral cortex is controversial.

Nerve growth factor treatment in dementia

Millions of people are affected by Alzheimer disease. As longevity increases, so will the number of patients with dementia. This has led to an intense search for successful treatment strategies. One area of interest is neurotrophic factors. Brain development and neuronal maintenance, as well as protective efforts, are mediated by a large number of different neurotrophic factors acting on specific receptors. In neurodegenerative disorders, there may be a possibility of rescuing degenerating neurons and stimulating terminal outgrowth with use of neurotrophic factors. The first neurotrophic factor discovered was nerve growth factor (NGF). A wealth of animal studies have shown that cholinergic neurons are NGF sensitive and NGF dependent, which is especially interesting in cognitive disorders, in which central cholinergic projections are important for cognitive function. In Alzheimer disease, cholinergic neurons have been shown to degenerate. This suggests that NGF may be used to pharmacologically counteract cholinergic degeneration and/or induce terminal sprouting in Alzheimer disease. Data from animal studies, as well as from the author's recent clinical trial, in which NGF was infused to the lateral ventricle in patients with Alzheimer disease, will be presented. Effects of NGF on cognition, as well as issues regarding dosage, side effects, and alternative ways of administering NGF, will be discussed.

NGF-mediated alteration of NF-kappaB binding activity after partial immunolesions to rat cholinergic basal forebrain neurons

There are age-associated cognitive and cholinergic deficits in the neurotrophin-dependent cholinergic basal forebrain neurons (CBFNs). There are also increases in the activity of the transcription factor NF-kappaB in the aged rodent brain that may reflect chronic enhancement of stress response signaling. We used partial immunolesions (PIL) to CBFN to examine the role of endogenous NGF on choline acetyltransferase (ChAT) activity and NGF-mediated NF-kappaB alteration after cholinergic deafferentation. We injected 192 IgG-saporin, an immunotoxin selectively taken up by neurotrophin receptor p75(NTR)-bearing neurons, into lateral ventricles, followed by infusions of anti-NGF to assess NF-kappaB, ChAT and NGF responses to PIL after anti-NGF infusion. Treatment with anti-NGF decreased ChAT activity by 17-34% in the cortex, hippocampus, and olfactory bulb and PIL decreased ChAT activity by 47-73%. Changes in AChE activity levels paralleled those observed for ChAT after PIL. NGF protein levels in the olfactory bulb, but not the cortex or hippocampus, increased significantly after PIL treatment. Infusion of anti-NGF abolished the PIL-induced eight-fold NGF increase in CNS. NF-kappaB binding activity to the IgG-kappaB and ChAT specific NF-kappaB consensus sequences, increased in the cortex but not hippocampus after PIL followed by anti-NGF infusion. It is likely that immunolesion-induced changes in ambient NGF levels may perturb NF-kappaB activity.

Repeated immunolesions display diminished stress response signal

Cholinergic basal forebrain neurons (CBFNs) retrogradely transport neurotrophins released in the hippocampus and cortex as part of a general response to injury in a process that is impaired in the aged rodent and can be spared by the exogenous addition of pharmacological doses of nerve growth factor (NGF). This observation suggests that components of stress response signal transduction pathways in the aged CNS can be exogenously activated. The extent and mechanism of the endogenous stimulation of NGF in response to injury can be mimicked via treatment with 192 IgG-saporin of rat CNS, an immunolesion model. Here we report on the use of a conditioning lesion paradigm to determine if repeated partial immunolesions have a conditioning effect on the immunolesion-induced increases in NGF protein or decreases in choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity. We report that chronic repeated immunolesions, as used here, were not as effective as a one time equivalent immunolesion in terms of induced NGF protein increases or decreasing ChAT and AChE activity in the hippocampus and cortex. Thus, chronic lesions resulting in cholinergic impairment typical of the aged CNS may differ from acute toxic models as a result of desensitization due to a conditioning effect of chronic subthreshold lesioning events in the CNS.

Macrophage cell-conditioned medium promotes cholinergic differentiation of undifferentiated progenitors and synergizes with nerve growth factor action in the developing basal forebrain

Conditioned medium from stimulated microglia and from the monocyte/macrophage cell line (RAW 264.7; MC-CM) promotes the differentiation of cholinergic neurons from undifferentiated progenitors in the septal nuclei and adjacent basal forebrain (BF). We have studied the regulation of this process by measuring the activity of choline acetyltransferase (ChAT) in cultured BF taken from embryonic day 16 rat brain. Inhibition of either xanthine oxidase with allopurinol or nitric oxide synthase with N(G)-monomethyl-l-arginine produces a small but significant improvement in the efficacy of MC-CM while inclusion of pyrrolidine dithiocarbamate, a hydroxyl radical scavenger widely used as an antioxidant, lowers MC-CM-induced ChAT activity. Addition of nerve growth factor (NGF) but not brain-derived neurotrophic factor or glial-derived neurotrophic factor together with MC-CM has a synergistic effect on both ChAT activity and ChAT mRNA, raising ChAT activity as much as 29-fold and ChAT mRNA almost 15-fold. While MC-CM raised mRNA for trkA, the effect was not synergistic with NGF. mRNA for the common neurotrophin receptor (p75NTR) showed a modest synergistic increase. Blockade of the Ras/Raf/ERK [extracellular signal-regulated kinase, also known as mitogen-activated protein [(MAP) kinase] signal transduction pathway with either PD28059 (an inhibitor of MAP kinase/ERK kinase kinase or MEK) or N-acetyl-S-farnesyl-l-cysteine (an inhibitor of Ras farnesylation and, hence, activation) inhibited the action of MC-CM. Moreover, a subpopulation of cells responded rapidly to MC-CM with an increased appearance of phosphorylated ERK. Because NGF also utilizes this pathway, synergy may occur along this signal transduction pathway.

Promoter-activated expression of nerve growth factor for treatment of neurodegenerative diseases

Genetic transfer approaches have received recent consideration as potential treatment modalities for human central and peripheral nervous system (CNS and PNS, respectively) neurodegenerative disorders, including Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis. Transplantation of genetically modified cells into the brain represents a promising strategy for the delivery and expression of specific neurotrophic factors, neurotransmitter-synthesizing enzymes, and cellular regulatory proteins for intervention in neurodegenerative diseases. The use of specific regulatable promoters may also provide potential control of gene expression required for dose-specific or time-specific therapeutic strategies. In this article, we review the potential use of activated promoters in ex vivo systems for the potential genetic therapy of neurodegenerative disorders, and then describe our own studies using the zinc-inducible metallothionein promoter for the regulated expression of nerve growth factor (NGF) in rodent brain transplants.

Liposome-mediated NGF gene transfection following neuronal injury: potential therapeutic applications

We have systematically investigated the therapeutic potential of cationic liposome-mediated neurotrophic gene transfer for treatment of CNS injury. Following determination of optimal transfection conditions, we examined the effects of dimethylaminoethane-carbamoyl-cholesterol (DC-Chol) liposome-mediated NGF cDNA transfection in injured and uninjured primary septo-hippocampal cell cultures and rat brains. In in vitro studies, we detected an increase of NGF mRNA in cultures 1 day after transfection. Subsequent ELISA and PC12 cell biological assays confirmed that cultured cells secreted soluble active NGF into the media from day 2 after gene transfection. Further experiments showed that such NGF gene transfection reduced the loss of chol- ine acetyltransferase (ChAT) activity in cultures following calcium-dependent depolarization injury. In in vivo studies, following intraventricular injections of NGF cDNA complexed with DC-Chol liposomes, ELISA detected nine- to 12-fold increases of NGF in rat CSF. Further studies showed that liposome/NGF cDNA complexes could attenuate the loss of cholinergic neuronal immunostaining in the rat septum after traumatic brain injury (TBI). Since deficits in cholinergic neurotransmission are a major consequence of TBI, our studies demonstrate for the first time that DC-Chol liposome-mediated NGF gene transfection may have therapeutic potential for treatment of brain injury.

Effect of NGF treatment on outcome measures in a rat model of middle cerebral artery occlusion

Ischemic insults to the brain result in a time-dependent increase in neuronal death that is responsible for some of the functional deficits associated with stroke. Our working hypothesis is that ischemia results in a prompt depletion of high energy phosphate species resulting in decreased pH and glutathione levels in brain in a temporal and spatial pattern that disrupts nerve growth factor homeostasis and increases neuronal apoptosis. Here we show hemispheric depletion of active phosphate species after ischemia. Also, we observed that the striatum is an early target for oxidative stress that is followed by energy metabolic impairment and altered neurotrophin levels that were detected by noninvasive magnetic resonance imaging (MRI) measurements of cytotoxicity and conventional biochemical determinations of apoptosis, glutathione, and nerve growth factor (NGF) protein levels in a pattern distinct from that observed in the hippocampus. Furthermore, early assessment of intracellular pH by 31P-magnetic resonance spectroscopy (31P-MRS) was a predictor of later infarct development as determined by MRI. We also show that pretreatment with pharmacological doses of NGF did not have overall significant beneficial consequences on irreversible ischemia in an intraluminal unilateral irreversible model of stroke in rat brain.

Herpes simplex virus type 1 vector-mediated expression of nerve growth factor protects dorsal root ganglion neurons from peroxide toxicity

Nerve growth factor beta subunit (beta-NGF) transgene delivery and expression by herpes simplex virus type 1 (HSV-1) vectors was examined in a cell culture model of neuroprotection from hydrogen peroxide toxicity. Replication-competent (tk- K mutant background) and replication-defective (ICP4(-);tk- S mutant background) vectors were engineered to contain the murine beta-NGF cDNA under transcriptional control of either the human cytomegalovirus immediate-early gene promoter (HCMV IEp) (e.g., KHN and SHN) or the latency-active promoter 2 (LAP2) (e.g., KLN and SLN) within the viral thymidine kinase (tk) locus. Infection of rat B103 and mouse N2A neuronal cell lines, 9L rat glioma cells, and Vero cells with the KHN or SHN vectors resulted in the production of beta-NGF-specific transcripts and beta-NGF protein reaching a maximum at 3 days postinfection (p.i.). NGF protein was released into the culture media in amounts ranging from 10.83 to 352.86 ng/ml, with the highest levels being achieved in B103 cells, and was capable of inducing neurite sprouting of PC-12 cells. The same vectors produced high levels of NGF in primary dorsal root ganglion (DRG) cultures at 3 days. In contrast to HCMV IEp-mediated expression, the LAP2-NGF vectors showed robust expression in primary DRG neurons at 14 days. The neuroprotective effect of vector produced NGF was assessed by its ability to inhibit hydrogen peroxide-induced neuron toxicity in primary DRG cultures. Consistent with the kinetics of vector-mediated NGF expression, HCMV-NGF vectors were effective in abrogating the toxic effects of peroxide at 3 but not 14 days p.i. whereas LAP2-NGF vector transduction inhibited apoptosis in DRG neurons at 14 days p.i. but was ineffective at 3 days p.i. Similar kinetics of NGF expression were observed with the KHN and KLN vectors in latently infected mouse trigeminal ganglia, where high levels of beta-NGF protein expression were detected at 4 wks p.i. only from the LAP2; HCMV-NGF-driven expression peaked at 3 days but could not be detected during HSV latency at 4 weeks. Together, these results indicate that (i) NGF vector-infected cells produce and secrete mature, biologically active beta-NGF; (ii) vector-synthesized NGF was capable of blocking peroxide-induced apoptosis in primary DRG cultures; and (iii) the HCMV-IEp functioned to produce high levels of NGF for several days; but (iv) only the native LAP2 was capable of long-term expression of a therapeutic gene product in latently infected neurons in vivo.

Simulation of cortical cholinergic deficits--a novel experimental approach to study pathogenetic aspects of Alzheimer's disease

Cholinergic lesion paradigms have been used to study the role of the cholinergic system in cognitive function, and its implication in cognitive deficits that occur in Alzheimer's disease. In the last few years an increasing number of studies have applied neurotoxins including excitotoxins or cholinotoxins to produce reductions in cortical cholinergic activity. One of the most serious limitations of these lesion paradigms is the fact that the cytotoxins used are far from being selective to cholinergic cells. Recently, a monoclonal antibody to the low-affinity nerve growth factor (NGF) receptor, 192IgG, coupled to a cytotoxin, saporin, has been described as an efficient and selective immunotoxin for the NGF-receptor bearing cholinergic neurons in rat basal forebrain. Here we demonstrate the usefulness of 192IgG-saporin as a powerful tool for producing an animal model with selective and specific basal forebrain cholinergic lesions in rats which can be applied to simulate some neurochemical sequelae of Alzheimer's disease including cholinergic mechanisms in processing of the amyloid precursor protein, and could be of particular value to elaborate and to test therapeutical strategies compensating for the reduced cortical cholinergic input.

Induction of apoptosis in the CNS during development by the combination of hyperoxia and inhibition of glutathione synthesis

Apoptosis in the central nervous system (in contrast to necrosis) is an endogenous cell suicide mechanism triggered in response to biological factors and genotoxic stimuli often resulting from oxidative stress. Excessive neural apoptosis may result in longterm brain dysfunction. A significant proportion of prematurely born infants are exposed to high oxygen and nutritional regimens deficient in antioxidant precursors. Such infants frequently display cognitive deficits when studied in later childhood. Studies in cell culture have characterized a close relationship between oxidative stress, glutathione availability and cell death. Here, we assessed this relationship in rat brain, as a model approximation of the situation that occurs in human infants. Two day old rats were exposed to an atmosphere of 95% oxygen and treated with buthionine sulfoximine (BSO), a glutathione synthesis inhibitor. Control groups consisted of rat-pups kept in air, air plus BSO, or oxygen alone. At the end of 5 days of treatment, brains were harvested, dissected and nerve growth factor protein (NGF), glutathione, and extent of apoptosis were measured. Hyperoxia induced a decrease in NGF protein while BSO induced a decrease in glutathione concentrations. Animals treated with both hyperoxia and BSO had a dramatic increase in the extent of brain apoptosis detected. We conclude from these studies that the brains of animals exposed to both oxidative stress and limited antioxidant protection are liable to pro-apoptotic changes. Increased cell death via apoptosis reflecting changes in neurotrophin and glutathione homeostasis may represent the mechanism responsible for the induction of the longterm cognitive deficits observed in some preterm infants.

Selective in vivo fluorescence labelling of cholinergic neurons containing p75(NTR) in the rat basal forebrain

The cholinergic system of the rat basal forebrain is used as a model for the homologous region in humans which is highly susceptible to neuropathological alterations as in Alzheimer's disease. Cholinergic cells in the basal forebrain express the low-affinity neurotrophin receptor p75NTR. This has been utilized for selective immunolesioning of cholinergic neurons after internalization of an immunotoxin composed of anti-p75NTR and the ribosome-inactivating toxin saporin. However, the goal of many studies may be not the lesion, but the identification of cholinergic cells after other experimentally induced alterations in the basal forebrain. Therefore, a novel cholinergic marker was prepared by conjugating the monoclonal antibody 192IgG directed against p75NTR with the bright red fluorochrome carbocyanine 3 (Cy3). Three days after intraventricular injection of Cy3-192IgG the fluorescence microscopic analysis revealed a pattern of Cy3-labelled cells matching the distribution of cholinergic neurons. Apparently the marker was internalized within complexes of p75NTR and Cy3-192IgG which were then retrogradely transported to the cholinergic perikarya of the basal forebrain. In addition to the even labelling of somata, a strong punctate-like Cy3-immunofluorescence was seen in structures resembling lysosomes. The specificity of the in vivo staining was proven by subsequent immunolabelling of choline acetyltransferase (ChAT) with green fluorescent Cy2-tagged secondary antibodies. In the medial septum, the diagonal band and the nucleus basalis only cholinergic neurons were marked by Cy3-192IgG. In parallel experiments, digoxigenylated 192IgG was not detectable within cholinergic basal forebrain neurons after intraventricular injection. Presumably, this modified antibody could not be internalized. On the other hand, digoxigenylated 192IgG was found to be an excellent immunocytochemical marker for p75NTR as shown by double labelling including highly sensitive mouse antibodies directed against ChAT. Based on the present findings, future applications of the apparently non-toxic Cy3-192IgG and other antibodies for fluorescent in vivo and in vitro labelling are discussed.

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.

Long term changes in brain cholinergic markers and nerve growth factor levels after partial immunolesion

There are deficits in cholinergic basal forebrain neurons (CBFNs) in the aged brain and patients suffering Alzheimer's disease associated with a partial loss of the CBFNs. To mimic this partial loss and assess its long term effects on residual cholinergic activity and resultant target-derived nerve growth factor (NGF) levels, we produced a partial immunolesion to CBFNs with 192 IgG-saporin, an immunotoxin selectively taken up by p75NTR-bearing neurons. We measured two cholinergic markers, choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activity, and NGF protein levels at 10 days, 1, 6 and 12 months postlesion. There were no significant changes in the cholinergic markers and the NGF protein levels in the sham-treated animal controls during the one year experiment. Ten days after 192 IgG-saporin treatment, ChAT activity decreased to 35-50% of controls in the olfactory bulb, hippocampus, and cortex. There was a minor but significant recovery of ChAT activity one year after the immunolesion in the hippocampus. Changes in AChE activity mirrored the ChAT changes but were less robust. There were transient increases in NGF protein levels in the hippocampus and cortex that returned to basal levels at 6 months and 12 months postlesion, respectively. In summary, partial immunolesions resulted in partial region-specific and time-dependent recoveries of cholinergic activity in the target areas of the basal forebrain after a partial elimination of CBFNs and a return to basal levels of NGF protein consistent with the hypothesis that the remaining CBFNs compensated for losses of ChAT and NGF due to changes in cholinergic innervation of basal forebrain target areas.

Interleukin-6 is not expressed in activated microglia and in reactive astrocytes in response to lesion of rat basal forebrain cholinergic system as demonstrated by combined in situ hybridization and immunocytochemistry

Interleukin-6 may play an essential role in early inflammatory processes as response to degenerating cholinergic cells in the nucleus basalis of Meynert in patients suffering Alzheimer's disease. The cholinergic immunotoxin, 192IgG-saporin, was applied to produce selective and specific degenerations of basal forebrain cholinergic cells. To disclose the lesion-induced temporal cascade of the expression pattern of IL-6, and to reveal the cellular source for production and secretion of IL-6 in vivo after endogeneously induced basal forebrain cholinergic cell loss, both in situ hybridization and immunocytochemistry for IL-6 were performed. To identify the cell types expressing IL-6 mRNA, double labeling techniques were applied combining in situ hybridization technique with immunocytochemistry and lectin histochemistry for both micro- and astroglia and a number of neuronal markers including choline acetyltransferase, parvalbumin, and neurofilaments. In the intact brain, IL-6 is mainly localized in neurons, in particular in both cholinergic and GABAergic neurons of the basal forebrain. Although basal forebrain cholinergic lesion resulted in a dramatic increase in the number of micro- and astroglial cells at the lesion site, IL-6 expression could not be detected in any of the lesion-induced activated glial cell types. Moreover, cholinergic lesion led to a reduced number of IL-6-expressing cells in the basal forebrain, which is assumed to be due to the loss of cholinergic cells. The predominantly neuronal localization in rat brain suggests a role for IL-6 in activating micro- and astroglial cells in response to degenerating cholinergic neurons.

Cholinergic immunolesions by 192IgG-saporin--useful tool to simulate pathogenic aspects of Alzheimer's disease

Alzheimer's disease, the most common cause of senile dementia, is characterized by intracellular formation of neurofibrillary tangles, extracellular deposits of beta amyloid as well as cerebrovascular amyloid accumulation and a profound loss of cholinergic neurons within the nucleus basalis Meynert with alterations in cortical neurotransmitter receptor densities. The use of the cholinergic immunotoxin 192IgG-saporin allows for the first time study of the impact of cortical cholinergic deafferentation on cortical neurotransmission, learning, and memory without direct effects on other neuronal systems. This model also allows the elucidation of contributions of cholinergic mechanisms to the establishment of other pathological features of Alzheimer's disease. The findings discussed here demonstrate that cholinergic immunolesions by 192IgG-saporin induce highly specific, permanent cortical cholinergic hypoactivity and alterations in cortical neurotransmitter densities comparable to those described for Alzheimer's disease. The induced cortical cholinergic deficit also leads to cortical/hippocampal neurotrophin accumulation and reduced amyloid precursor protein (APP) secretion, possibly reflecting the lack of stimulation of postsynaptic M1/M3 muscarinic receptors coupled to protein kinase C. This immunolesion model should prove useful to test therapeutic strategies based on stimulation of cortical cholinergic neurotransmission or amelioration of pathogenic aspects of cholinergic degeneration in the basal forebrain. Application of the model to animal species that can develop beta-amyloid plaques could provide information about the contribution of cholinergic function to amyloidogenic APP processing.

In vivo regulation of amyloid precursor protein secretion in rat neocortex by cholinergic activity

The proteolytic cleavage of the amyloid precursor protein (APP) has been shown to be modulated through specific muscarinic receptor activation in vitro in both transfected cell lines and native brain slices, whereas a demonstration of receptor-mediated control of APP processing under in vivo conditions is still lacking. To simulate alterations in muscarinic receptor stimulation in vivo, we have (i) specifically reduced the cortical cholinergic innervation in rats using partial immunolesions with 192IgG-saporin, and (ii) restored cholinergic function in lesioned rats by transplantation of nerve growth factor producing fibroblasts. While total APP levels in cortical homogenates were unaffected by cholinergic deafferentation, we observed a significant reduction in the abundance of secreted APP and a concomitant increase in membrane-bound APP. These changes were reversed in immunolesioned rats with nerve growth factor-producing fibroblasts. There was a strong positive correlation between the ratio of secreted APP to membrane-bound APP and the activity of choline acetyltransferase and M1 muscarinic acetylcholine receptor density (measured by [3H]pirenzepine binding) in experimental groups. Additionally, we observed a transient decrease in the ratio of cortical APP transcripts containing the Kunitz protease inhibitor domain (APP 770 and APP 751) versus APP 695 in rats with cholinergic hypoactivity. The data presented suggest that cortical APP processing is under basal forebrain cholinergic control, presumably mediated through M1 muscarinic acetylcholine receptors on cholinoceptive cortical target cells.

Basal forebrain cholinergic immunolesion by 192IgG-saporin: evidence for a presynaptic location of subpopulations of alpha 2- and beta-adrenergic as well as 5-HT2A receptors on cortical cholinergic terminals

To study whether the changes in cortical noradrenergic and serotonergic mechanisms observed in patients with Alzheimer's disease are the consequence of reduced cortical cholinergic activity, a novel colinergic immunotoxin (conjugate of the monoclonal antibody 192IgG against the lower affinity nerve growth factor receptor with the cytotoxic protein saporin, 192IgG-saporin) was used to produce a specific and selective loss of cholinergic cells in rat basal forebrain nuclei. To correlate the responses to cholinergic immunolesion in cholinoceptive cortical target regions with cholinergic hypoactivity, quantitative receptor autoradiography to measure adrenoceptors and 5-hydroxytryptamine (5-HT) receptor subtypes, and histochemistry to estimate acetylcholinesterase activity, were performed in adjacent brain sections. alpha 1-adrenoceptor and 5-HT1A receptor binding were not affected by cholinergic immunolesion in any of the cortical and hippocampal regions studied. However, cholinergic immunolesion resulted in significantly reduced alpha 2- and beta-adrenoceptor as well as 5-HT2A receptor binding in a number of cortical and hippocampal regions displaying a reduced activity of acetylcholinesterase, already detectable seven days after a single injection of 192IgG-saporin and persisting up to three months post lesion without any significant recovery. The data suggest that at least a subpopulation of alpha 2- and beta-adrenoceptor as well 5-HT2A receptor subtype is present on cortical and hippocampal cholinergic terminals originating in the basal forebrain. The lesion-induced receptor changes suggest that the alterations in cortical 5-HT2 receptor binding observed in patients with Alzheimer's disease might be secondary to cholinergic deficits.

Electron microscopic evidence for microglial phagocytic activity and cholinergic cell death after administration of the immunotoxin 192IgG-saporin in rat

192IgG-saporin represents a novel cholinergic immunotoxin which selectively and specifically destroys cholinergic cells in rat basal forebrain. Activated microglial cells are known to play an important role in phagocytosis in regions of neuronal loss. To study the immunotoxin-induced phagocytic events in the basal forebrain activated microglial cells were visualized by lectin cytochemistry using Griffonia simplicifolia agglutinin and analyzed by electron microscopy. Three and 7 days following an intracerebro-ventricular injection of 4 microg 192IgG-saporin, increased numbers of activated microglial cells were observed at both survival times, but the number was strikingly increased at day 7 postlesion. Three days after immunotoxin application microglial cells displayed features similar to those of resting microglia. Only translucent vacuole-like hollows were found intracellularly beneath the plasma membrane of microglial cells and in the adjoining extracellular space. Most neurons in the vicinity of microglial cells did not show any signs of degeneration. However, 7 days after injection of the immunotoxin microglial cells revealed different stages of phagocytosis. The majority of microglial cells were localized in perineuronal positions attached by processes to large areas of neuronal soma or dendrites, which in general showed signs of severe degeneration. The present study provides electron microscopic evidence for phagocytic microglial reactions in the rat basal forebrain after cholinergic lesion by 192IgG-saporin.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

Expression of amyloid precursor protein mRNA isoforms in rat brain is differentially regulated during postnatal maturation and by cholinergic activity

Pathological processing of the amyloid precursor protein (APP) is assumed to be responsible for the amyloid deposits in Alzheimer-diseased brain tissue, but the physiological function of this protein in the brain is still unclear. The aim of this study is to reveal whether the expression of different splicing variants of APP transcripts in distinct brain regions is driven by postnatal maturation and/or regulated by cortical cholinergic transmission, applying quantitative in situ hybridization histochemistry using 35S-labeled oligonucleotides as specific probes to differentiate between APP isoforms. In cortical brain regions, the expression of both APP695 and APP751 is high at birth and exhibits nearly adult levels. The developmental expression pattern of cortical APP695 displays a peak value around postnatal day 10, while the age-related expression of APP751 demonstrates peak values on postnatal days 10 and 25, with the highest steady state levels of APP751 mRNA on day 25. During early development, the cortical laminar distribution of the APP695, but not APP751, mRNA transiently changes from a more homogeneous distribution at birth to a pronounced laminar pattern with higher mRNA levels in cortical layer III/IV detectable at the age of 4 days and persisting until postnatal day 10. The distinct age-related changes in cortical APP695 and APP751 mRNA levels reflect the functional alterations during early brain maturation and suggest that APP695 might play a role in establishing the mature connectional pattern between neurons, whereas APP751 could play a role in controlling cellular growth and synaptogenesis. Lesion of basal forebrain cholinergic system by the selective cholinergic immunotoxin 192IgG-saporin resulted in decreased levels of APP695 but not APP751 and APP770 transcripts by about 15-20% in some cortical (cingulate, frontal, parietal, piriform cortex), hippocampal regions (CA1, dentate gyrus), and basal forebrain nuclei (medial septum, vertical limb of diagonal band), detectable not earlier than 30 days after lesion and persisting until 90 days postlesion, suggesting that the nearly complete loss of cortical cholinergic input does not have any significant impact on the expression of APP mRNA isoforms in cholinoceptive cortical target regions.