Gene transfer provides a practical means for safe, long-term, targeted delivery of biologically active neurotrophic factor proteins for neurodegenerative diseases

Efforts to develop neurotrophic factors to restore function and protect dying neurons in chronic neurodegenerative diseases like Alzheimer's (AD) and Parkinson's (PD) have been attempted for decades. Despite abundant data establishing nonclinical proof-of-concept, significant delivery issues have precluded the successful translation of this concept to the clinic. The development of AAV2 viral vectors to deliver therapeutic genes has emerged as a safe and effective means to achieve sustained, long-term, targeted, bioactive protein expression. Thus, it potentially offers a practical means to solve those long-standing delivery/translational issues associated with neurotrophic factors. Data are presented for two AAV2 viral vector constructs expressing one of two different neurotrophic factors: nerve growth factor (NGF) and neurturin (NRTN). One (AAV2-NGF; aka CERE-110) is being developed as a treatment to improve the function and delay further degeneration of cholinergic neurons in the nucleus basalis of Meynert, the degeneration of which has been linked to cognitive deficits in AD. The other (AAV2-NRTN; aka CERE-120) is similarly being developed to treat the degenerating nigrostriatal dopamine neurons and major motor deficits in PD. The data presented here demonstrate: (1) 2-year, targeted, bioactive-protein in monkeys, (2) persistent, bioactive-protein throughout the life-span of the rat, and (3) accurately targeted bioactive-protein in aged rats, with (4) no safety issues or antibodies to the protein detected. They also provide empirical guidance to establish parameters for human dosing and collectively support the idea that gene transfer may overcome key delivery obstacles that have precluded successful translation of neurotrophic factors to the clinic. More specifically, they also enabled the AAV-NGF and AAV-NRTN programs to advance into ongoing multi-center, double-blind clinical trials in AD and PD patients.

Advancing neurotrophic factors as treatments for age-related neurodegenerative diseases: developing and demonstrating "clinical proof-of-concept" for AAV-neurturin (CERE-120) in Parkinson's disease

Neurotrophic factors have long shown promise as potential therapies for age-related neurodegenerative diseases. However, 20 years of largely disappointing clinical results have underscored the difficulties involved with safely and effectively delivering these proteins to targeted sites within the central nervous system. Recent progress establishes that gene transfer can now likely overcome the delivery issues plaguing the translation of neurotrophic factors. This may be best exemplified by adeno-associated virus serotype-2-neurturin (CERE-120), a viral-vector construct designed to deliver the neurotrophic factor, neurturin to degenerating nigrostriatal neurons in Parkinson's disease. Eighty Parkinson's subjects have been dosed with CERE-120 (some 7+ years ago), with long-term, targeted neurturin expression confirmed and no serious safety issues identified. A double-blind, controlled Phase 2a trial established clinical "proof-of-concept" via 19 of the 24 prescribed efficacy end points favoring CERE-120 at the 12-month protocol-prescribed time point and all but one favoring CERE-120 at the 18-month secondary time point (p = 0.007 and 0.001, respectively). Moreover, clinically meaningful benefit was seen with CERE-120 on several specific protocol-prescribed, pairwise, blinded, motor, and quality-of-life end points at 12 months, and an even greater number of end points at 18 months. Because the trial failed to meet the primary end point (Unified Parkinson's Disease Rating Scale motor-off, measured at 12 months), a revised multicenter Phase 1/2b protocol was designed to enhance the neurotrophic effects of CERE-120, using insight gained from the Phase 2a trial. This review summarizes the development of CERE-120 from its inception through establishing "clinical proof-of-concept" and beyond. The translational obstacles and issues confronted, and the strategies applied, are reviewed. This information should be informative to investigators interested in translational research and development for age-related and other neurodegenerative diseases.

Translating the therapeutic potential of neurotrophic factors to clinical 'proof of concept': a personal saga achieving a career-long quest

While the therapeutic potential of neurotrophic factors has been well-recognized for over two decades, attempts to translate that potential to the clinic have been disappointing, largely due to significant delivery obstacles. Similarly, gene therapy (or gene transfer) emerged as a potentially powerful, new therapeutic approach nearly two decades ago and despite its promise, also suffered serious setbacks when applied to the human clinic. As advances continue to be made in both fields, ironically, they may now be poised to complement each other to produce a translational breakthrough. The accumulated data argue that gene transfer provides the 'enabling technology' that can solve the age-old delivery problems that have plagued the translation of neurotrophic factors as treatments for chronic central nervous system diseases. A leading translational program applying gene transfer to deliver a neurotrophic factor to rejuvenate and protect degenerating human neurons is CERE-120 (AAV2-NRTN). To date, over two dozen nonclinical studies and three clinical trials have been completed. A fourth (pivotal) clinical trial has completed all dosing and is currently evaluating safety and efficacy. In total, eighty Parkinson's disease (PD) subjects have thus far been dosed with CERE-120 (some 7 years ago), representing over 250 cumulative patient-years of exposure, with no serious safety issues identified. In a completed sham-surgery, double-blinded controlled trial, though the primary endpoint (the Unified Parkinson's Disease Rating Scale (UDPRS) motor off score measured at 12 months) did not show benefit from CERE-120, several important motor and quality of life measurements did, including the same UPDRS-motor-off score, pre-specified to also be measured at a longer, 18-month post-dosing time point. Importantly, not a single measurement favored the sham control group. This study therefore, provided important, well-controlled evidence establishing 'clinical proof of concept' for gene transfer to the CNS and the first controlled evidence for clinical benefit of a neurotrophic factor in a human neurodegenerative disease. This paper reviews the development of CERE-120, starting historically with the long-standing interest in the therapeutic potential of neurotrophic factors and continuing with selective accounts of past efforts to translate their potential to the clinic, eventually leading to the application of gene transfer and its role as the 'enabling technology'. Because of growing interest in translational R&D, including its practice in industry, the paper is uniquely oriented from the author's personal, quasi-autobiographic perspective and career-long experiences conducting translational research and development, with a focus on various translational neurotrophic factor programs spanning 30+ years in Big Pharma and development-stage biotech companies. It is hoped that by sharing these perspectives, practical insight and information might be provided to others also interested in translational R&D as well as neurotrophic factors and gene therapy, offering readers the opportunity to benefit from some of our successes, while possibly avoiding some of our missteps.

Delivery of hydrophobised 5-fluorouracil derivative to brain tissue through intravenous route using surface modified nanogels

Random copolymeric micelles composed of N-isopropylacrylamide (NIPAAM) and N-vinylpyrrolidone (VP) cross-linked with N,N'-methylenebisacrylamide (MBA) have been used as nanogel carriers to encapsulate N-hexylcarbamoyl-5-fluorouracil (HCFU), a prodrug of 5-FU, and have been targeted to brain tissue across blood-brain barrier (BBB) after coating with polysorbate 80. Accumulation of nanogel particles in the brain and other tissues of "strain A" mice had been monitored by radiolabeling of nanogels with (99m)Tc. Gamma Scintigraphic technique was also performed to see the distribution of (99m)Tc labeled nanogels in the brain. The retention time in blood appeared to be slightly longer for coated nanogels than that of uncoated nanogels though the accumulation of coated nanogels in the RES was more or less same as that of uncoated nanogels. The blood however had almost double accumulation of polysorbate 80 coated nanogels in the initial 5 min compared to that shown by uncoated nanogels. We speculate that coating of nanogels with polysorbate 80 alters the surface properties of nanogels, which results in relatively higher uptake in the brain tissue. The studies revealed that a large portion of (99m)Tc labeled HCFU loaded nanogels are accumulated in the RES (lung, liver and spleen). The accumulation of the labeled nanogels in the brain, however, is much less compared to RES and it has been found that while an amount of uncoated labeled nanogels was found to be 0.18% of the injected dose, it increased to 0.52% on coating with polysorbate 80. The optimal amount of polysorbate 80 added to nanogels for the maximum delivery of particles to brain was found to be 1% w/w. These results were further supported by the gamma scintigrams of New Zealand rabbits. Thus, the present nanogel system has opened a new avenue for poorly soluble drugs to be targeted to brain by coating the particles with polysorbate 80.

Gambogic amide, a selective agonist for TrkA receptor that possesses robust neurotrophic activity, prevents neuronal cell death

Nerve growth factor (NGF) binds to TrkA receptor and triggers activation of numerous signaling cascades, which play critical roles in neuronal plasticity, survival, and neurite outgrowth. To mimic NGF functions pharmacologically, we developed a high-throughput screening assay to identify small-molecule agonists for TrkA receptor. The most potent compound, gambogic amide, selectively binds to TrkA, but not TrkB or TrkC, and robustly induces its tyrosine phosphorylation and downstream signaling activation, including Akt and MAPKs. Further, it strongly prevents glutamate-induced neuronal cell death and provokes prominent neurite outgrowth in PC12 cells. Gambogic amide specifically interacts with the cytoplasmic juxtamembrane domain of TrkA receptor and triggers its dimerization. Administration of this molecule in mice substantially diminishes kainic acid-triggered neuronal cell death and decreases infarct volume in the transient middle cerebral artery occlusion model of stroke. Thus, gambogic amide might not only establish a powerful platform for dissection of the physiological roles of NGF and TrkA receptor but also provide effective treatments for neurodegenerative diseases and stroke.

Age-dependent loss of NGF signaling in the rat basal forebrain is due to disrupted MAPK activation

The loss of nerve growth factor (NGF) and its high affinity receptor TrkA has been implicated in the loss of cholinergic tone and function in Alzheimer's disease (AD) and normal aging. We employed an animal model of aging, the aged rat, which also exhibits memory loss and NGF alterations. Basal forebrain TrkA levels increased after injection of NGF in the hippocampus within 1h in young rats, but this response was diminished in aged animals as determined by Western blot analysis. Further, NGF activated MAPK pathways without changing total ERK levels and the activation of these pathways was also diminished in aged animals. The exogenous NGF injection did not appear to activate the PI-3K pathway or alter total levels of Akt significantly. These data shed light on mechanisms of NGF signaling in the CNS, and alterations in this signaling cascade associated with age and memory loss. These findings might lead to development of novel treatment therapies for the memory loss associated with AD and other age-associated neurodegenerative diseases.

Nerve Growth Factor Differentially Affects Spatial and Recognition Memory in Aged Rats

In rats, object discrimination depends on the integrity of the cholinergic system, thus it could be expected that nerve growth factor (NGF) can improve the behavior in aged subjects. The interactive effect of age and cholinergic improvement was assessed behaviorally in young and aged rats. Animals were injected by infusion of NGF into the lateral ventricles and they were tested in two behavioral tasks: an object-location and an object-recognition task. Spatial and recognition memory were assessed in an open field containing five different objects. Rats were submitted to six consecutive sessions. Both age-groups showed comparable habituation of exploratory response in Session 1-4. Discrimination index (DI) was calculated to assess responses to spatial change in Session 5 and object change in Session 6. Control young and aged rats were able to discriminate between familiar and novel object, however DI was lower in aged rats. Treatment with NGF induced decline of object discrimination in both age-groups. Different results were obtained in spatial displacement test. NGF was able to improve spatial memory in aged rats, but had no effect in young controls. These data confer on NGF potential role in improving spatial but not episodic memory in aged rats.

ERK-mediated NGF signaling in the rat septo-hippocampal pathway diminishes with age

RATIONALE

Degeneration of basal forebrain cholinergic neurons (BFCNs) plays an important role in aging and Alzheimer's disease (AD) pathology. This degeneration may be a result of disrupted nerve growth factor (NGF) signaling. Aged rats have memory deficits, BFCN degeneration, and disrupted NGF signaling.

OBJECTIVE

In this study we identify a rapid NGF signaling pathway in BFCNs and the second messenger system associated with that signaling. We also identify age-dependent alterations in this signaling pathway.

MATERIALS AND METHODS

After cognitive assessment using the Morris water maze, rats were given an intra-hippocampal NGF injection. Basal forebrain immunohistochemical analysis, confocal microscopy, and inhibitor studies were performed.

RESULTS

An increase in immunoreactivity for the NGF receptor TrkA was found in cell bodies of BFCNs 15 min and 1 h post-NGF injection. Immunohistochemistry studies with phospho-ERK and phospho-AKT antibodies showed that this rapid signaling occurred through MAP kinase, but not PI-3 kinase pathways. MAPK inhibitor studies attenuated the NGF-induced effects. Both TrkA and phospho-ERK (extracellular signal-regulated kinase) immunoreactivities were diminished in aged rats and phospho-ERK immunoreactivity-correlated with aged rat performance in the Morris water maze.

CONCLUSIONS

Rapid NGF signaling likely occurs in the rat CNS through the MAPK signaling pathway. This rapid signaling pathway is diminished in aged rats compared to young ones and may contribute to memory deficits observed in aged rats. As cholinergic degeneration coupled with altered levels of NGF and TrkA receptors are also seen in human aging and AD, ERK-related dysfunction may be relevant in human conditions as well.

The possible role of neurotrophins in the pathogenesis and therapy of schizophrenia

The pathogenesis of schizophrenia may be ascribed to early maldevelopment of brain tissue. Neurotrophins are a group of dimeric proteins that affect the development of the nervous system in all vertebrates' species. Since neurotrophins, as well as other growth factors, play a crucial role in neurodevelopment, they are plausible candidates of taking part in the pathophysiology of schizophrenia. In line with this hypothesis, accumulating preclinical and clinical data indicate that dysfunctions of nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) may contribute to impaired brain development, neuroplasticity and synaptic "dysconnectivity" leading to the schizophrenic syndrome, or at least some of its presentations. This article reviews the functions of neurotrophins in the complex process of normal brain development, and their possible relevance to the neuropathology and neuropharmacology of schizophrenia. Further research in this area may bring about novel pharmacological therapeutic strategies to this chronic debilitating disorder.

Age-related sex differences in spatial learning and basal forebrain cholinergic neurons in F344 rats

Basal forebrain cholinergic neurons are important for spatial learning in rodents. Spatial learning ability is reportedly better in males than females, and declines with age. To examine the role of cholinergic function in sex- or age-related differences in spatial learning, we compared the size of basal forebrain cholinergic neurons (BFCN) of young and aged male and female Fischer 344 (F344) rats that had been trained in the Morris water maze. Young male and female rats were equally proficient in finding the platform during training trials, but probe tests revealed that young male rats had better knowledge of the platform's precise location. Impairments in spatial learning were observed in aged rats, and the advantage of males over females was lost. BFCN were significantly larger in young male than young female rats, and were correlated with spatial memory performance for both groups. BFCN were smaller in aged than young males; no change was seen between young and aged females. In the groups of aged rats the correlation between neuron size and spatial memory was lost. The present findings provide further evidence of a role for the basal forebrain cholinergic system in spatial learning, but reveal a complex interaction between sex, age and behavioral performance.

Exogenous NGF restores endogenous NGF distribution in the brain of the cognitively impaired aged rat

Alzheimer's disease and normal aging may impair retrograde transport of nerve growth factor (NGF) from cortical areas to basal forebrain cholinergic neurons. We demonstrate a relationship between performance in a spatial reference memory task and NGF distribution in the aged rat brain. In addition, exogenous NGF restored endogenous NGF distribution in cognitively impaired aged rats. These data suggest that NGF administration restores utilization of endogenous growth factor in the brain of cognitively impaired aged rats.

Frontal cortex BDNF levels correlate with working memory in an animal model of Down syndrome

Individuals with Down syndrome (DS) develop most neuropathological hallmarks of Alzheimer's disease early in life, including loss of cholinergic markers in the basal forebrain. Ts65Dn mice, an animal model of DS, perform poorly on tasks requiring spatial memory and also exhibit basal forebrain pathology beginning around 6 months of age. We evaluated memory as well as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) protein levels in basal forebrain, frontal cortex, hippocampus, and striatum in Ts65Dn mice at the age when cholinergic degeneration is first observed, and compared values to normosomic controls. Six-month-old Ts65Dn mice exhibited impairments in working and reference memory as assessed on a water radial-arm maze. The working memory deficit was related to the inability of Ts65Dn mice to successfully sustain performance as the working memory load increased. Coupled with cognitive performance deficiencies, Ts65Dn mice also exhibited lower frontal cortex BDNF protein levels than controls. Further, BDNF levels were negatively correlated with working memory errors during the latter portion of testing in Ts65Dn mice, thereby suggesting that lower BDNF protein levels in the frontal cortex may be associated with the observed working memory impairment.

Environmental influences on brain neurotrophins in rats

Environmental factors can have profound influences on the brain. Enriching environments with physical, social and sensory stimuli are now established to be beneficial to brain development and ageing. A multitude of responses from cellular and molecular mechanisms to macroscopic changes in neural morphology and neurogenesis have been considered in the context for evidences that environmental inputs can regulate brain plasticity in the rat at all stages of life. Data from our laboratory have revealed that enriched environment increased nerve growth factor (NGF) gene expression and protein levels in the hippocampus, and this may contribute to events underlying environmentally induced neural plasticity. Because neurotrophic factors are essential for neural development and survival, they are likely to be involved in the cerebral consequences modified by enriched experiences.

Hippocampal tyrosine kinase A receptors are restricted primarily to presynaptic vesicle clusters

Adult septohippocampal cholinergic neurons are dependent on trophic support for normal functioning and survival; these effects are largely mediated by the tyrosine kinase A receptor (TrkA), which binds its ligand, nerve growth factor (NGF), with high affinity. To determine the subcellular localization of TrkA within septohippocampal terminal fields, two rabbit polyclonal antisera to the extracellular domain of TrkA were localized immunocytochemically in rat dentate gyrus by light and electron microscopy. By light microscopy, TrkA immunoreactivity was found mostly in fine, varicose fibers primarily in the hilus and, to a lesser extent, in the granule cell and molecular layers. By electron microscopy, the central and infragranular regions of the hilus contained the highest densities of TrkA-immunoreactive profiles. Most TrkA-labeled profiles were axons (31% of 3,473), axon terminals (20%), and glia (38%); fewer were dendrites (6%), dendritic spines (5%), and granule cell and interneuron somata (<1%). TrkA immunolabeling in axons and axon terminals was discrete, often concentrated in patches of small synaptic vesicles that were adjacent to somatic and dendritic profiles. TrkA-labeled terminals formed both asymmetric and symmetric synapses, primarily with dendritic shafts and spines. TrkA-immunoreactive glial profiles frequently apposed terminals contacting dendritic spines. The findings that presynaptic profiles contain TrkA immunolabeling in sites of vesicle accumulation suggest that NGF binding to TrkA may influence transmitter release. The presence of TrkA immunoreactivity in somata, dendrites, and glia further suggests that cells within the dentate gyrus may take up NGF.

Intranigral transplantation of solid tissue ventral mesencephalon or striatal grafts induces behavioral recovery in 6-OHDA-lesioned rats

Parkinson's disease (PD) is characterized by a degeneration of the dopamine (DA) pathway from the substantia nigra (SN) to the basal forebrain. Prior studies in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats have primarily concentrated on the implantation of fetal ventral mesencephalon (VM) into the striatum in attempts to restore DA function in the target. We implanted solid blocks of fetal VM or fetal striatal tissue into the SN to investigate whether intra-nigral grafts would restore motor function in unilaterally 6-OHDA-lesioned rats. Intra-nigral fetal striatal and VM grafts elicited a significant and long-lasting reduction in apomorphine-induced rotational behavior. Lesioned animals with ectopic grafts or sham surgery as well as animals that received intra-nigral grafts of fetal cerebellar cortex showed no recovery of motor symmetry. Subsequent immunohistochemical studies demonstrated that VM grafts, but not cerebellar grafted tissue expressed tyrosine hydroxylase (TH)-positive cell bodies and were associated with the innervation by TH-positive fibers into the lesioned SN as well as adjacent brain areas. Striatal grafts were also associated with the expression of TH-positive cell bodies and fibers extending into the lesioned SN and an induction of TH-immunolabeling in endogenous SN cell bodies. This finding suggests that trophic influences of transplanted fetal striatal tissue can stimulate the re-expression of dopaminergic phenotype in SN neurons following a 6-OHDA lesion. Our data support the hypothesis that a dopaminergic re-innervation of the SN and surrounding tissue by a single solid tissue graft is sufficient to improve motor asymmetry in unilateral 6-OHDA-lesioned rats.

Long-term environmental enrichment leads to regional increases in neurotrophin levels in rat brain

A number of studies have demonstrated that both morphological and biochemical indices in the brain undergo alterations in response to environmental influences. In previous work we have shown that rats raised in an enriched environmental condition (EC) perform better on a spatial memory task than rats raised in isolated conditions (IC). We have also found that EC rats have a higher density of immunoreactivity than IC rats for both low and high affinity nerve growth factor (NGF) receptors in the basal forebrain. In order to determine if these alterations were coupled with altered levels of neurotrophins in other brain regions as well, we measured neurotrophin levels in rats that were raised in EC or IC conditions. Rats were placed in the different environments at 2 months of age and 12 months later brain regions were dissected and analyzed for NGF, brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) levels using Promega ELISA kits. We found that NGF and BDNF levels were increased in the cerebral cortex, hippocampal formation, basal forebrain, and hindbrain in EC animals compared to age-matched IC animals. NT-3 was found to be increased in the basal forebrain and cerebral cortex of EC animals as well. These findings demonstrate significant alterations in NGF, BDNF, and NT-3 protein levels in several brain regions as a result of an enriched versus an isolated environment and thus provide a possible biochemical basis for behavioral and morphological alterations that have been found to occur with a shifting environmental stimulus.

Sex- and age-related P75 neurotrophin receptor expression in the human supraoptic nucleus

The human supraoptic nucleus (SON) is the main production site of plasma vasopressin. Previously, using the Golgi apparatus and cell size as measures for neuronal metabolic activity, an activation of vasopressinergic neurons was found during ageing in the human SON in women but not in men. We hypothesized that the low-affinity neurotrophin receptor p75 (p75(NTR)) might be involved in the mechanism of activation of vasopressin neurons in postmenopausal women, since this receptor was found to be expressed in the SON neurons of aged individuals, and because p75(NTR) expression was shown to be suppressed by estrogens. Therefore, we investigated whether p75(NTR) immunoreactivity in the SON neurons was age- and sex-dependent. For this purpose, we studied paraffin sections of the SON in 32 postmortem brains of control patients ranging in age from 29 to 94 years with an anti-p75(NTR) antibody and determined the area of p75(NTR) immunoreactivity per neuron using an image analysis system. To study whether the p75(NTR) might also participate in the activation of SON neurons, we related Golgi apparatus size to the area of p75(NTR) immunoreactivity per cell in the same patients. We found that the area of p75(NTR) immunoreactivity per cell correlated indeed significantly with age and with Golgi apparatus size only in women but not in men. Therefore, our results suggest that p75(NTR) is involved in postmenopausal activation of vasopressinergic neurons in the human SON.

Changes in brain nerve growth factor levels and nerve growth factor receptors in rats exposed to environmental enrichment for one year

This study examined the effects of long-term differential rearing on levels of brain nerve growth factor, its receptors, and their relationships to cognitive function. Adult rats (two months old) were placed into either enriched or standard housing conditions where they remained for 12 months. Animals from the enriched condition group had significantly higher levels of nerve growth factor in hippocampus, visual and entorhinal cortices compared with animals housed in isolated condition. Immunohistochemical analysis of brain tissue from the medial septal area revealed higher staining intensity and fibre density with both the low-affinity and the high-affinity nerve growth factor receptors. Enriched rats performed better than isolated rats in acquisition of spatial learning and had lower locomotion scores in the open field. These results provide further evidence that experimental stimulation results in increased production of trophic factors and structural reorganization in specific brain regions known to be involved in cognitive function.

Acute application of NGF increases the firing rate of aged rat basal forebrain neurons

Nerve growth factor (NGF) has been widely used in animal models to ameliorate age-related neurodegeneration, but it cannot cross the blood-brain barrier (BBB). NGF conjugated to an antibody against the transferrin receptor (OX-26) crosses the BBB and affects the biochemistry and morphology of NGF-deprived basal forebrain neurons. The rapid actions of NGF, including electrophysiological effects on these neurons, are not well understood. In the present study, two model systems in which basal forebrain neurons either respond dysfunctionally to NGF (aged rats) or do not have access to target-derived NGF (intraocular transplants of forebrain neurons) were tested. One group of transplanted and one group of aged animals received unconjugated OX-26 and NGF comixture as a control, while other groups received replacement NGF in the form of OX-26-NGF conjugate during the 3 months preceding the electrophysiological recording session. Neurons from animals in both the transplanted and aged control groups showed a significant increase in firing rate in response to acute NGF application, while none of the conjugate-treated groups or young intact rats showed any response. After the recordings, forebrain transplants and aged brains were immunocytochemically stained for the low-affinity NGF receptor. All conjugate treatment groups showed significantly greater staining intensity compared to controls. These data from both transplants and aged rats in situ indicate that NGF-deprived basal forebrain neurons respond to acute NGF with an increased firing rate. This novel finding may have importance even for long-term biological effects of this trophic factor in the basal forebrain.

Septal cholinergic deafferentation of the dentate gyrus results in a loss of a subset of neuropeptide Y somata and an increase in synaptic area on remaining neuropeptide Y dendrites

Removal of cholinergic septal inputs using the immunotoxin 192 IgG-saporin reduces the number of interneurons containing neuropeptide Y (NPY)-immunoreactivity in the rat dentate gyrus by approximately 30% [Milner et al., J. Comp. Neurol. 386 (1997) 48-59]. The goal of the present study was to determine if NPY-containing neurons that survive deafferentation have any distinguishing morphological and/or microenvironmental features. For this, 2 or 24 weeks after intracerebroventricular injections of 192 IgG-saporin, NPY-immunolabeled neurons in the hilus of the dentate gyrus were examined by electron microscopy. Neither the size nor morphological traits of NPY-labeled perikaryal or dendritic profiles from lesioned compared to control rats at either time-point differed significantly. However, at both time-points, NPY-containing somatal profiles from immunolesioned rats compared to controls had a reduced percentage of their plasmalemmal surface apposed to unmyelinated axon profiles and an increased percentage of their surface occupied by astrocytic profiles. At the 24 week time-point, these differences were statistically significant. The primary contributing factor for these changes was the absence of a subgroup of NPY-labeled somatal profiles in lesioned rats compared to controls which was: (a) distinguished by frequent appositions of unmyelinated axons (from 15 to 35%) to the plasmalemmal surface; and (b) located primarily in the central hilar region. Unlike NPY-containing somata, changes associated with NPY-labeled dendritic profiles were exclusively related to associated presynaptic profiles at the 24 week time-point. In lesioned rats compared to controls at this time-point, NPY-containing dendritic profiles had a concurrent increase in the percentage of the plasmalemmal surface occupied by active zones and the size of terminals contacting them. The present results combined with those of our earlier study suggest that septal cholinergic deafferentation results in: (a) the loss of a distinct subpopulation of hippocampal NPY-containing neurons; and (b) an increase in total active zone area suggesting a strengthening of synaptic connections to the surviving population of NPY-containing neurons in the long term.

Apoptosis in neuronal development and transplantation: role of caspases and trophic factors

Fetal ventral mesencephalic (VM) transplants have been studied in the context of dopaminergic (DA) replacement therapy for Parkinson's disease (PD). DA neurons from VM transplants will grow axons and form functional synapses in the adult host central nervous system (CNS). Recently, studies have demonstrated that most of the transplanted DA neurons die in grafts within the first week after implantation. An important feature of neural development, also in transplanted developing fetal neural tissue, is cell death. However, while about 50% of cells born in the CNS will die naturally, up to 99% of fetal cells die after neural transplantation. It has been shown that VM grafts contain many apoptotic cells even at 14 days after transplantation. The interleukin-1beta converting enzyme (ICE) cysteine protease and 11 other ICE-like-related proteases have been identified, now named caspases. Activation of caspases is one of the final steps before a neuron is committed to die by apoptosis. Here we review this cell death process in detail: Since the growth of fetal neural grafts placed in the adult brain in many ways mimics normal development, it is likely that the caspases also play a functional role in transplants. Pharmacological inhibitors of caspases and genetically modified mice are now available for the study of neuronal death in fetal neuronal transplants. Understanding cell death mechanisms involved in acute cellular injury, necrosis, and programmed cell death (PCD) is useful in improving future neuronal transplantation methodology, as well as in neuroprotection, for patients with neurodegenerative diseases.

Behavioral effects of neurotrophic factor supplementation in aging

Neurotrophic factors are now recognized to play important roles in the normal function of the mature central nervous system. This knowledge has motivated experiments to evaluate the potential benefits of administering neurotrophic factors to the aged brain. This article provides a review of studies to date that have determined the behavioral effects of such treatments. Nerve growth factor (NGF) administration appears to reliably enhance learning and memory in aged rats, while glial-derived neurotrophic factor (GDNF) causes some improvement in motor function. Problems associated with neurotrophic factor administration to humans are discussed.

Effects of osteogenic protein-1 (OP-1) treatment on fetal spinal cord transplants to the anterior chamber of the eye

Spinal cord injury represents a serious medical problem, and leads to chronic conditions that cannot be reversed at present. It has been suggested that trophic factor treatment may reduce the extent of damage and restore damaged neurons following the injury. We have tested the effects of osteogenic protein-1 (OP-1, also known as BMP-7), a member of the transforming growth factor-beta superfamily of growth factors, on developing spinal cord motor neurons in an intraocular transplantation model. Embryonic day 13 or 18 spinal cord tissue was dissected, incubated with OP-1 or vehicle, and injected into the anterior chamber of the eye of adult rats. Injections of additional doses of OP-1 were performed weekly, and the overall growth of the grafted tissue was assessed noninvasively. Four to 6 weeks postgrafting, animals were sacrificed and the tissue was processed for immunohistochemistry using antibodies directed against choline acetyltransferase, neurofilament, and the dendritic marker MAP-II. We found that OP-1 treatment stimulated overall growth of spinal cord tissue when dissected from embryonic day 18, but not from embryonic day 13. OP-1 treatment increased cell size and extent of cholinergic markers in motor neurons from both embryonic stages. The neurons also appeared to have a more extensive dendritic network in OP-1-treated grafts compared to controls. These findings indicate that OP-1 treatment may reduce the extent of axotomy-induced cell death of motor neurons, at least in the developing spinal cord.

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.

A non-invasive system for delivering neural growth factors across the blood-brain barrier: a review

Intraventricular administration of nerve growth factor (NGF) in rats has been shown to reduce age-related atrophy of central cholinergic neurons and the accompanying memory impairment, as well as protect these neurons against a variety of perturbations. Since neurotrophins do not pass the blood-brain barrier (BBB) in significant amounts, a non-invasive delivery system for this group of therapeutic molecules needs to be developed. We have utilized a carrier system, consisting of NGF covalently linked to an anti-transferrin receptor antibody (OX-26), to transport biologically active NGF across the BBB. The biological activity of this carrier system was tested using in vitro bioassays and intraocular transplants; we were able to demonstrate that cholinergic markers in both developing and aged intraocular septal grafts were enhanced by intravenous delivery of the OX-26-NGF conjugate. In subsequent experiments, aged (24 months old) Fischer 344 rats received intravenous injections of the OX-26-NGF conjugate for 6 weeks, resulting in a significant improvement in spatial learning in previously impaired rats, but disrupting the learning ability of previously unimpaired rats. Neuroanatomical analyses showed that OX-26-NGF conjugate treatment resulted in a significant increase in cholinergic cell size as well as an upregulation of both low and high affinity NGF receptors in the medial septal region of rats initially impaired in spatial learning. Finally, OX-26-NGF was able to protect striatal cholinergic neurons against excitotoxicity and basal forebrain cholinergic neurons from degeneration associated with chemically-induced loss of target neurons. These results indicate the potential utility of the transferrin receptor antibody delivery system for treatment of neurodegenerative disorders with neurotrophic substances.

The role of neuronal growth factors in neurodegenerative disorders of the human brain

Recent evidence suggests that neurotrophic factors that promote the survival or differentiation of developing neurons may also protect mature neurons from neuronal atrophy in the degenerating human brain. Furthermore, it has been proposed that the pathogenesis of human neurodegenerative disorders may be due to an alteration in neurotrophic factor and/or trk receptor levels. The use of neurotrophic factors as therapeutic agents is a novel approach aimed at restoring and maintaining neuronal function in the central nervous system (CNS). Research is currently being undertaken to determine potential mechanisms to deliver neurotrophic factors to selectively vulnerable regions of the CNS. However, while there is widespread interest in the use of neurotrophic factors to prevent and/or reduce the neuronal cell loss and atrophy observed in neurodegenerative disorders, little research has been performed examining the expression and functional role of these factors in the normal and diseased human brain. This review will discuss recent studies and examine the role members of the nerve growth factor family (NGF, BDNF and NT-3) and trk receptors as well as additional growth factors (GDNF, TGF-alpha and IGF-I) may play in neurodegenerative disorders of the human brain.