Recovery of nucleus basalis cholinergic neurons by grafting NGF secretor fibroblasts

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

The use of nonneuronal cells for gene delivery

The implantation of genetically engineered nonneuronal cells can provide an effective method for achieving localized delivery of discrete molecules to the CNS or for providing substrates for regrowth of neural structures. Most primary nonneuronal cells have the advantage of being easily obtainable from the prospective host for ex vivo retrovirus-mediated genetic manipulation (most will be mitotic in culture) and reimplantation as an autologous graft (circumventing the problem of immune rejection). As primary cells, they are unlikely to be tumorigenic. The most vexing problem for such systems remains the apparent loss of transgene expression from viral promoters after prolonged periods of engraftment. Much effort is currently being directed at optimizing sustained transgene expression by varying the promoters, by varying the cell types to be engineered, or by regulating expression by enhancing promoter function or substrate availability. While nonneuronal cells are excellent vehicles for achieving passive delivery of substances to the CNS, they lack the ability to incorporate into the host cytoarchitecture in a functional manner (e.g., make synaptic contacts). For this reason, not only may certain essential circuits not be re-formed, but the regulated release of certain substances through feedback loops may be missing. While apparently unimportant for some substances (e.g., ACh), for others (e.g., NGF), their unregulated, inappropriate, excessive, or ectopic release may actually be inimical to the host. Furthermore, the loss of foreign gene expression (the bane of gene therapy) may leave engineered nonneural cells incapacitated, whereas donor tissue originating from brain may intrinsically produce various CNS factors allowing correction to proceed despite inactivation of the introduced gene. In fact, CNS-derived tissue may provide as-yet-unrecognized endogenous neuralspecific substances which are equally as beneficial to the host as the gene in question. Thus, future developments in gene delivery to the brain for some conditions may emphasize using neurons or neural progenitors for ex vivo genetic manipulation (Fisher, 1997) and refining techniques for the direct injection of therapeutic genes into neurons in vivo (see Snyder and Fisher, 1996). For a wide variety of conditions, however, using nonneuronal cellular vehicles or even nonbiologic synthetic vehicles may be efficient, effective, and safe strategies for the passive delivery of therapeutic molecules to discrete regions of the CNS. In fact, this approach may come closer than any other to immediate human applications.

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

The aim of the present study was to examine the effects of nerve growth factor on brain cholinergic function after a partial immunolesion to the rat cholinergic basal forebrain neurons (CBFNs) by 192 IgG-saporin. Two weeks after intraventricular injections of 1.3 micrograms of 192 IgG-saporin, about 50% of CBFNs were lost which was associated with 40-60% reductions of choline acetyltransferase (ChAT) and high-affinity choline uptake (HACU) activities throughout the basal forebrain cholinergic system. Two groups of lesioned animals received intraventricular transplantations of mouse 3T3 fibroblasts retrovirally transfected with either the rat NGF gene (3T3NGF+) or the retrovirus alone (3T3NGF-) and were sacrificed eight weeks later. In vivo production of NGF by 3T3NGF+ cells was confirmed by NGF immunohistochemistry on the grafts and NGF immunoassay on cerebrospinal fluid (CSF) samples. Both ChAT and HACU activities returned to normal control levels in the basal forebrain and cortex after 3T3NGF+ transplants, whereas no recovery was observed in 3T3NGF- transplanted animals. There was a 25% increase in the size of remaining CBFNs and an increased staining intensity for NGF immunoreactivity in these cells after NGF treatments. Acetylcholinesterase (AChE) histochemistry revealed that the optical density of AChE-positive fibers in the cerebral cortex and hippocampus were reduced by about 60% in immunolesioned rats which were completely restored by 3T3NGF+ grafts. In addition, decreases in growth-associated protein (GAP)-43 immunoreactivity after immunolesion and increases in synaptophysin immunoreactivity after 3T3NGF+ grafts were observed in the hippocampus. Our results further confirm the notion that transfected NGF-secreting cells are useful in long-term in vivo NGF treatment and NGF can upregulate CBFN function. They also highly suggest that NGF induces terminal sprouting from remaining CBFNs.

Effects of chronic infusion of nerve growth factor (NGF) in rats with nucleus basalis magnocellularis lesion

We attempted to evaluate the effects of bilateral injection of ibotenic acid (IA) into the nucleus basalis magnocellularis (nbm) of rats as well as the potential recovery mediated by the infusion of nerve growth factor (NGF). The lesion caused an impairment of learning and memory processes. Also, a severe depletion of choline acetyl transferase activity was detected in cortical areas. After the NGF administration, a significant reversion of these functional changes was observed. Thus, IA-lesioned rats might serve as a model for the evaluation of neurotrophic factors actions on basal forebrain damaged neurons.

Microencapsulation and the grafting of genetically transformed cells as therapeutic strategies to rescue degenerating neurons of the CNS

A number of approaches have been developed to deliver growth factors within the central nervous system of adult mammals. Initially a variety of neurotrophic factors were administered either by single intracerebroventricular or local injections directly into brain tissues or via permanently installed cannulae for chronic administrations. More recently delivery systems including conjugates, biodegradable and non-biodegradable implants and microspheres as well as genetically engineered cells have been introduced in order to provide a prolonged supply of neurotrophic factors and to prevent their enzymatic degradation. In this review we examine a variety of means of delivering neurotrophic factors (mainly nerve growth factor) with the primary focus upon the use of microencapsulated neurotrophins and cells genetically modified to produce them. In addition, this review highlights some difficulties and future trends in the development of novel delivery systems hopefully more suitable for investigations in different areas of neuroscience.

Estrogen and nerve growth factor-related systems in brain. Effects on basal forebrain cholinergic neurons and implications for learning and memory processes and aging

Estrogen replacement can significantly affect the expression of ChAT and NGF receptors in specific basal forebrain cholinergic neurons. The time-course of the effects is consistent with a direct up-regulation of ChAT followed by either direct or indirect down-regulation of p75NGFR and trkA NGF receptors, possibly due to increased cholinergic activity in the hippocampal formation and cortex and a decrease in hippocampal levels of NGF. Current evidence suggests ChAT, p75NGFR, trkA, and NGF all play a role in regulating cholinergic function in the hippocampal formation and cortex. In addition, all have been implicated in the maintenance of normal learning and memory processes as well as in changes in cognitive function associated with aging and with neurodegenerative disease. It is possible that estrogen may affect cognitive function via effects on NGF-related systems and basal forebrain cholinergic neurons. Effects of estrogen on cognitive function have been reported, as has some preliminary evidence for beneficial effects of estrogen in decreasing the prevalence of and reducing some cognitive deficits associated with Alzheimer's disease. Whether these effects are related to effects on NGF-related systems or basal forebrain cholinergic neurons is currently unknown. Indirect evidence suggests that estrogen interacts with NGF-related systems and that changes in circulating levels of estrogen can contribute to age-related changes in hippocampal levels of NGF. These findings have important implications for consideration of estrogen replacement therapy in pre- and post-menopausal women. Further studies examining effects of different regimens of estrogen replacement as well as estrogen combined with progesterone on NGF and basal forebrain cholinergic neurons in young and aged animals are required. Prospective studies correlating aging and estrogen replacement with numbers of basal forebrain cholinergic neurons and hippocampal and cortical levels of NGF also need to be performed to better assess the potential benefits of estrogen replacement in reducing age- and disease-related cognitive decline.

Grafting of nerve growth factor-producing fibroblasts reduces behavioral deficits in rats with lesions of the nucleus basalis magnocellularis

Rats received bilateral lesions of the nucleus basalis magnocellularis by infusion of biotenic acid. Two weeks after the lesion, a suspension of genetically modified primary rat fibroblasts was grafted dorsal to the nucleus basalis magnocellularis (2 x 10(5) cells per side). The fibroblasts were either infected with the gene for human beta-nerve growth factor or Escherichia coli beta-galactosidase. The nerve growth factor-producing fibroblasts released 67 ng nerve growth factor/10(5) cells per day in vitro. Two weeks after implantation of the fibroblasts, spatial learning was tested in the Morris water-maze. Nerve growth factor-producing fibroblasts, but not beta-galactosidase-producing fibroblasts ameliorated the deficit in acquisition of the water-maze task. In addition, spatial acuity was improved to near-normal levels by the nerve growth factor-producing grafts. Choline acetyltransferase activity in cortical areas and hippocampus was not affected by the nerve growth factor-producing grafts. Both grafted groups showed a similar reduction in the level of dopamine, but not homovanillic acid or 3-methoxytyramine, in the frontal cortex. Levels of norepinephrine, epinephrine and serotonin and their metabolites in the neocortex and hippocampus were not affected by the lesion or the grafts. Nerve growth factor-producing grafts increased the size of remaining nerve growth factor-receptor (p75) immunoreactive neurons in the nucleus basalis magnocellularis by 25%. Nucleus basalis magnocellularis lesions reduced the integrated optic density of choline acetyltransferase-positive fiber staining in the ventral neocortex by 46%, but nerve growth factor-producing grafts restored this area to 86% of control. These data suggest that nerve growth factor-producing grafts can cause a marked behavioral improvement, probably through the partial restoration of the lesioned projection from nucleus basalis magnocellularis to neocortex.

Microencapsulation of genetically engineered fibroblasts secreting nerve growth factor

We demonstrated that genetically modified fibroblasts can be encapsulated into biocompatible, biodegradable spheres retaining their viability and capacity to continuously secrete nerve growth factor (NGF) for at least two months. Genetically engineered rat fibroblasts producing NGF were encapsulated in an alginate-polylysine gel with the ultimate objective of improving transplantation methodologies. Cultures were suspended in a sodium alginate solution and the suspension was extruded drop-wise into a solution of calcium chloride. Morphological properties of the spheres were assessed by light and electron microscopy. The spheres had a homogenous external membrane, without fibroblasts, protruding from the surface of the capsular membrane. The NGF determinations in culture media showed that encapsulated fibroblasts continued to synthesize NGF for at least 60 days. We also confirmed that secreted NGF was biologically active, by assessing the induction of choline acetyltransferase (ChAT) activity in dissociated embryonic rat septal cultures. These results encourage further studies using in vivo models to determine the value of applying microencapsulated genetically modified cells secreting trophic factors as a therapeutic strategy for central nervous system (CNS) injuries.

Grafting of genetically modified cells: effects of acetylcholine release in vivo

In this study, microdialysis was used to investigate functional recovery of central cholinergic neurons in the forebrain of rats with cortical devascularizing lesions. Mature male rats were unilaterally lesioned by disruption of the pia arachnoid vessels and genetically modified fibroblasts secreting nerve growth factor (NGF) were placed at the site of the lesion. One month following surgery, microdialysis probes were installed in the remaining cortex and were perfused with artificial cerebrospinal fluid (csf) containing neostigmine (5 nM) and/or KCl (100 mM). The basal (non-stimulated) release of acetylcholine (ACh) in the cortex was similar in all experimental groups, whereas KCl stimulated release of ACh was significantly augmented (P < 0.05) in the ipsilateral remaining cortex in lesioned animals that have been implanted with fibroblasts secreting NGF. These results suggest that NGF secreted by genetically engineered fibroblasts modulates neuroplasticity in the adult mammalian CNS and may favour recovery of cortical function following injury.