Neurotrophic agents may exacerbate the pathologic cascade of Alzheimer's disease

Evidence for a novel neuronal mechanism driving Alzheimer's disease, upstream of amyloid

This perspective offers an alternative to the amyloid hypothesis in the etiology of Alzheimer's disease (AD). We review evidence for a novel signaling mechanism based on a little-known peptide, T14. T14 could drive neurodegeneration as an aberrantly activated process of plasticity selective to interconnecting subcortical nuclei, the isodendritic core, where cell loss starts at the pre-symptomatic stages of the disease. Each of these cell groups has the capacity to form T14, which can stimulate production of p-Tau and β-amyloid, suggestive of an upstream driver of neurodegeneration. Moreover, results in an animal AD model show that antagonism of T14 with a cyclated variant, NBP14, prevents formation of β-amyloid, and restores cognitive function to that of wild-type counterparts. Any diagnostic and/or therapeutic strategy based on T14-NBP14 awaits validation in clinical trials. However, an understanding of this novel signaling system could bring much-needed fresh insights into the progression of cell loss underlying AD. HIGHLIGHTS: The possible primary mechanism of neurodegeneration upstream of amyloid. Primary involvement of selectively vulnerable subcortical nuclei, isodendritic core. Bioactive peptide T14 trophic in development but toxic in context of mature brain. Potential for early-stage biomarker to detect Alzheimer's disease. Effective therapeutic halting neurodegeneration, validated already in 5XFAD mice.

Characterization of a Bioactive Peptide T14 in the Human and Rodent Substantia Nigra: Implications for Neurodegenerative Disease

The substantia nigra is generally considered to show significant cell loss not only in Parkinson's but also in Alzheimer's disease, conditions that share several neuropathological traits. An interesting feature of this nucleus is that the pars compacta dopaminergic neurons contain acetylcholinesterase (AChE). Independent of its enzymatic role, this protein is released from pars reticulata dendrites, with effects that have been observed in vitro, ex vivo and in vivo. The part of the molecule responsible for these actions has been identified as a 14-mer peptide, T14, cleaved from the AChE C-terminus and acting at an allosteric site on alpha-7 nicotinic receptors, with consequences implicated in neurodegeneration. Here, we show that free T14 is co-localized with tyrosine hydroxylase in rodent pars compacta neurons. In brains with Alzheimer's pathology, the T14 immunoreactivity in these neurons increases in density as their number decreases with the progression of the disease. To explore the functional implications of raised T14 levels in the substantia nigra, the effect of exogenous peptide on electrically evoked neuronal activation was tested in rat brain slices using optical imaging with a voltage-sensitive dye (Di-4-ANEPPS). A significant reduction in the activation response was observed; this was blocked by the cyclized variant of T14, NBP14. In contrast, no such effect of the peptide was seen in the striatum, a region lacking the T14 target, alpha-7 receptors. These findings add to the accumulating evidence that T14 is a key signaling molecule in neurodegenerative disorders and that its antagonist NBP14 has therapeutic potential.

Effects of Intraventricular Encapsulated Hngf-Secreting Fibroblasts in Aged Rats

Exogenous NGF administered into the central nervous system (CNS) has been reported to improve cognitive function in aged rats. However, concerns have been expressed about the risks involved with supplying NGF to the CNS. In this study, baby hamster kidney cells (BHK) genetically modified to secrete human NGF (hNGF) were encapsulated in semipermeable membranes and implanted intraventricularly. ChAT/LNGFR-positive basal forebrain neurons were shown to atrophy and degenerate with age, especially in cognitively impaired rats. The encapsulated BHK-NGF cells produced less than 10% of doses previously reported to be effective, but this was sufficient to increase the size of ChAT/LNGFR-positive basal forebrain neurons in the aged and learning-impaired rats to the size of the neurons in young healthy rats. The hNGF from these encapsulated cells also improved performance in a repeated-acquisition version of the Morris water maze spatial learning task in learning-impaired 20.6- and 26.7- mo-old rats. Furthermore, there was no evidence that these doses of hNGF impaired Morris water maze performance in the youngest 3.3-5.4 mo rats, and analyses of mortality rates, body weights, somatosensory thresholds, potential hyperalgesia, and activity levels, suggested that these levels of exogenous hNGF are not toxic or harmful to aged rats. These results suggest that CNS-implanted semipermeable membranes, containing genetically modified xenogeneic cells continuously producing these levels of hNGF, attenuate age-related cognitive deficits in nonimmunosuppressed aged rats, and that both the surgical implantation procedure and long-term exposure to low doses of hNGF appear safe in aged rats.

Enhancement of the nonamyloidogenic pathway by exogenous NGF in an Alzheimer transgenic mouse model

Nerve growth factor (NGF) is an important nerve cell growth regulatory factor and has an indispensable role in the development, survival and regeneration of the cholinergic basal forebrain (CBF) neurons, and it has multiple targets when used for Alzheimer's Disease (AD) therapy. In this study, we observed whether NGF can affect cholinergic neurons to change amyloid-β precursor protein (APP) metabolism process and reduce amyloidosis in AD brains. NGF was administered intranasally to APP/PS1 double-transgenic mice for 14weeks. We observed an increase in APP695 and ADAM10 and a decrease in BACE1 and PS1 protein levels and, subsequently, a reduction in Aβ1-40 and Aβ1-42 levels and Aβ burden were present in NGF-treated mice brains, suggesting that NGF enhanced the APP nonamyloidogenic cleavage pathway and reduced the Aβ generation in the APP/PS1 transgenic mice brains.

Neurotrophin levels and behaviour in BALB/c mice: impact of intermittent exposure to individual housing and wheel running

This study assessed the effects of intermittent individual housing on behaviour and brain neurotrophins, and whether physical exercise could influence alternate individual-housing-induced effects. Five-week-old BALB/c mice were either housed in enhanced social (E) or standard social (S) housing conditions for 2 weeks. Thereafter they were divided into six groups and for 6 weeks remained in the following experimental conditions: Control groups remained in their respective housing conditions (E-control, S-control); enhanced individual (E-individual) and standard individual (S-individual) groups were exposed every other day to individual cages without running-wheels; enhanced running-wheel (E-wheel) and standard running-wheel (S-wheel) groups were put on alternate days in individual running-wheel cages. Animals were assessed for activity in an automated individual cage system (LABORAS) and brain neurotrophins analysed. Intermittent individual housing increased behavioural activity and reduced nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) levels in frontal cortex; while it increased BDNF level in the amygdala and BDNF protein and mRNA in hippocampus. Besides normalizing motor activity and regulating BDNF and NGF levels in hippocampus, amygdala and cerebellum, physical exercise did not attenuate reduction of cortical NGF and BDNF induced by intermittent individual housing. This study demonstrates that alternate individual housing has significant impact on behaviour and brain neurotrophin levels in mice, which can be partially altered by voluntary physical exercise. Our results also suggest that some changes in neurotrophin levels induced by intermittent individual housing are not similar to those caused by continuous individual housing.

Loss of basal forebrain P75(NTR) immunoreactivity in subjects with mild cognitive impairment and Alzheimer's disease

The long-held belief that degeneration of the cholinergic basal forebrain was central to Alzheimer's disease (AD) pathogenesis and occurred early in the disease process has been questioned recently. In this regard, changes in some cholinergic basal forebrain (CBF) markers (e.g. the high affinity trkA receptor) but not others (e.g., cortical choline acetyltransferase [ChAT] activity, the number of ChAT and vesicular acetylcholine transporter-immunoreactive neurons) suggest specific phenotypic changes, but not frank neuronal degeneration, early in the disease process. The present study examined the expression of the low affinity p75 neurotrophin receptor (p75(NTR)), an excellent marker of CBF neurons, in postmortem tissue derived from clinically well-characterized individuals who have been classified as having no cognitive impairment (NCI), mild cognitive impairment (MCI), and mild AD. Relative to NCI individuals, a significant and similar reduction in the number of nucleus basalis p75(NTR)-immunoreactive neurons was seen in individuals with MCI (38%) and mild AD (43%). The number of p75(NTR)-immunoreactive nucleus basalis neurons was significantly correlated with performance on the Mini-Mental State Exam, a Global Cognitive Test score, as well as some individual tests of working memory and attention. These data, together with previous reports, support the concept that phenotypic changes, but not frank neuronal degeneration, occur early in cognitive decline. Although there was no difference in p75(NTR) CBF cell reduction between MCI and AD, it remains to be determined whether these findings lend support to the hypothesis that MCI is a prodromal stage of AD.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Herpesvirus-mediated systemic delivery of nerve growth factor

Sustained systemic dissemination of therapeutic proteins from peripheral sites is an attractive prospect for gene therapy applications. Replication-defective genomic herpes simplex virus type 1 (HSV-1) vectors were evaluated for their ability to express nerve growth factor (NGF) as a model gene product both locally and systemically. Intra-articular inoculation of NGF expression vectors in rabbits resulted in significant increases in joint lavage and blood plasma NGF that persisted for 1 year. A rhesus macaque injected intra-articularly displayed a comparable increase in plasma NGF for at least 6 months, at which time the serum NGF levels of this animal were sufficient to cause differentiation of PC12 cells in culture, but not to increase footpad epidermis innervation. Long-term reporter transgene expression was observed primarily in ligaments, a finding confirmed by direct inoculation of patellar ligament. Patellar ligament inoculation with a NGF vector resulted in elevated levels of circulating NGF similar to those observed following intra-articular vector delivery. These results represent the first demonstration of sustained systemic release of a transgene product using HSV vectors, raising the prospect of new applications for HSV-1 vectors in the treatment of systemic disease.

Neurotrophic factor strategies for the treatment of Alzheimer disease

Cholinergic neurons in the nucleus basalis of Meynert are reduced early in the course of Alzheimer disease, and the dysfunction of cholinergic neurons is believed to be primarily responsible for cognitive deficits in the disease. Nerve growth factor has a trophic effect on cholinergic neurons and therefore may have some beneficial effects on the cognitive impairment observed in patients with Alzheimer disease. Experimental studies demonstrated that a continuous infusion of nerve growth factor into the cerebroventricle prevents cholinergic neuron atrophy after axotomy or associated with normal aging and ameliorates cognition impairment in these animals. A clinical study in three patients with Alzheimer disease revealed, however, that a long-term intracerebroventricular infusion of nerve growth factor may have certain potentially beneficial effects, but the continuous intracerebroventricular route of administration is also associated with negative side effects that appear to outweigh the positive effects. Several other strategies have been suggested to provide neurotrophic support to cholinergic neurons. In this article, we review the neurotrophic factor strategies for the treatment of Alzheimer disease.

Aberrant expression of nNOS in pyramidal neurons in Alzheimer's disease is highly co-localized with p21ras and p16INK4a

Aberrancies of growth and proliferation-regulating mechanisms might be critically involved in the processes of neurodegeneration in Alzheimer's disease (AD). Expression of p21ras and further downstream signalling elements involved in regulation of proliferation and differentiation as, for example, MEK, ERK1/2, cyclins, cyclin-dependent kinases and their inhibitors such as those of the p16INK4a family, are elevated early during the course of neurodegeneration. Activation of p21ras can also directly be triggered by nitric oxide (NO), synthesized in the brain by various isoforms of nitric oxide synthase (NOS) that might be differentially involved into the pathomechanism of AD. To study the potential link of NO and critical regulators of cellular proliferation and differentiation in the process of neurofibrillary degeneration, we analyzed the expression pattern of NOS-isoforms, p21ras and p16INK4a compared to neurofibrillary degeneration in AD. Additionally to its expression in a subtype of cortical interneurons that contain the nNOS-isoform also in normal brain, nNOS was detected in pyramidal neurons containing neurofibrillary tangles or were even unaffected by neurofibrillary degeneration. Expression of nNOS in these neurons was highly co-localized with p21ras and p16INK4a. Because endogenous NO can activate p21ras in the same cell which in turn leads to cellular activation and stimulation of NOS expression [H.M. Lander, J.S. Ogiste, S.F.A. Pearce, R. Levi, A. Novogrodsky, Nitric oxide-stimulated guanine nucleotide exchange on p21 ras, J. Biol. Chem. 270 (1995) 7017-7020], the high level of co-expression of NOS and p21ras in neurons vulnerable to neurofibrillary degeneration early in the course of AD thus provides the basis for an autocrine feedback mechanism that might exacerbate the progression of neurodegeneration in a self-propagating manner.

P75 neurotrophin receptor in the nucleus basalis of meynert in relation to age, sex, and Alzheimer's disease

In a previous study we showed that the staining of tyrosine kinase receptors (trks), which are high-affinity neurotrophin receptors (NTRs), is strongly diminished in the nucleus basalis of Meynert (NBM) of Alzheimer's disease (AD) patients, which may explain the lack of effect of NGF therapy in AD patients so far. Since the literature regarding the expression of low-affinity NTRs was rather controversial, the aim of the present study was to examine (i) possible changes in the staining of low-affinity NTRs, i.e., p75 in the human NBM, an area that is severely affected in AD; and (ii) alterations of these receptors in relation to risk factors for AD, e. g., age, sex, and menopause. Brain material of 31 controls and 30 AD patients was obtained at autopsy, embedded in paraffin, and stained immunocytochemically. Using an image analysis system, we quantified p75 immunoreactivity in both cell bodies and fibers at the level of the NBM. Our results showed a significant diminishment of p75 immunoreactivity in both cell bodies and fibers of NBM neurons in AD. We did not find any relationship between age or sex and the expression of p75 receptor in cell bodies. However, there was a clearly positive relationship between age and fiber staining in AD patients which suggests the occurrence of a p75 transport disorder as an early event in the process of AD. These observations and the earlier reported decreased staining of trk receptors show that degeneration of NBM neurons in AD is associated with a decreased neurotrophin responsiveness of NBM neurons in AD and that therapeutic strategies should be directed toward upregulation of receptors or facilitation of transport before an effect of neurotrophins in AD may be expected.

Death of PC12 cells and hippocampal neurons induced by adenoviral-mediated FAD human amyloid precursor protein gene expression

We used adenoviral-mediated gene transfer of human amyloid precursor proteins (h-APPs) to evaluate the role of various h-APPs in causing neuronal cell death. We were able to infect PC12 cells with very high efficiency because approximately 90% of the cells were cytochemically positive for beta-galactosidase activity when an adenoviral vector containing LacZ cDNA was used to infect cells. Cells infected with adenovirus containing h-APP cDNA showed high-level transcription and expression of h-APP as measured by reverse transcriptase-polymerase chain reaction and Western immunoblot analyses, respectively. Intracellular and extracellular levels of h-APP were elevated approximately 17-and 24-fold in cultures infected with recombinant adenovirus containing wild-type mutant and 13- and 17-fold with V642F mutant. No elevation in h-APP was seen in cultures infected with antisense h-APP or null adenovirus. H-APP levels were maximal 3 days after infection. Overexpression of V642F mutant h-APP in PC12 cells and hippocampal neurons resulted in about a twofold increase in death compared with overexpression of wild-type h-APP. These results demonstrate the usefulness of recombinant adenoviral mediated gene transfer in cell culture studies and suggest that overexpression of a familial Alzheimer's disease mutant APP may be toxic to neuronal cells.

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.

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.

Sympathetic neurite outgrowth is greater on plaque-poor vs. plaque-rich regions of Alzheimer's disease cryostat sections

Senile plaques are a characteristic histopathological feature of Alzheimer's disease (AD) and are associated with altered neuritic morphology. Numerous individual plaque components, most notably beta-amyloid, have been studied for their possible effects on neurite outgrowth in culture. However, the effect of senile plaques on neuronal morphology and function is difficult to assess. In the present study, the effect of senile plaques on neurite outgrowth was studied by culturing embryonic chick sympathetic neuronal explants on Alzheimer's tissue sections. Explants were cultured for 3 days on amygdala tissue sections from AD as well as non-AD patients in serum-free medium. Neurite outgrowth on plaque-rich regions was compared with outgrowth on plaque-poor regions of the same tissue section, and with outgrowth on non-AD tissue, through colocalization of the living explants and the underlying plaques. Explants growing on plaque-rich regions showed significantly less neurite outgrowth compared with those on plaque-poor regions in the same section or on control brain tissue. These results suggest that plaques are poor substrates for neurite outgrowth as compared with non-plaque areas of the same tissue sections, and support the hypothesis that components of the senile plaques may inhibit neurite outgrowth.

Plastic neuronal remodeling is impaired in patients with Alzheimer's disease carrying apolipoprotein epsilon 4 allele

A relationship between the apolipoprotein E (apoE) genotype and the risk to develop Alzheimer's disease has been established recently. Apolipoprotein synthesis is implicated in developmental processes and in neuronal repair of the adult nervous system. In the present study, we investigated the influence of the apolipoprotein polymorphism on the severity of neuronal degeneration and the extent of plastic dendritic remodeling in Alzheimer's disease. Changes in length and arborization of dendrites of Golgi-impregnated neurons in the basal nucleus of Meynert, locus coeruleus, raphe magnus nucleus, medial amygdaloid nucleus, pedunculopontine tegmental nucleus, and substantia nigra were analyzed after three-dimensional reconstruction. Patients with either one or two apoE epsilon 4 alleles not only showed a more severe degeneration in all areas investigated than in patients lacking the apoE 4 allele but also revealed significantly less plastic dendritic changes. ApoE epsilon 4 allele copy number, furthermore, had a significant effect on the pattern of dendritic arborization. Moreover, the relationship between the intensity of dendritic growth and both the extent of neuronal degeneration and the stage of the disease seen in patients lacking the apoE epsilon 4 allele was very weak in the presence of one epsilon 4 allele and completely lost in patients homozygous for the epsilon 4 allele. The results provide direct evidence that neuronal reorganization is affected severely in patients with Alzheimer's disease carrying the apoE epsilon 4 allele. This impairment of neuronal repair might lead to a more rapid functional decompensation, thereby contributing to an earlier onset and more rapid progression of the disease.

Trk receptor alterations in Alzheimer's disease

The expression of trk receptors in postmortem normal, Huntington's disease and Alzheimer's disease human brains was investigated using immunohistochemistry, in-situ hybridisation and Western blotting. Alzheimer's disease hippocampi displayed an increase in trkA receptor levels in astrocytes in the CA1 region, some of which were associated with beta-amyloid-positive plaques. Truncated trkB receptors were found in high levels in senile plaques, while the full-length receptor was expressed in glial-like cells in the hippocampus of Alzheimer's disease brains. In-situ hybridisation studies indicated that trk receptor mRNA was also elevated in Alzheimer's. The appearance of trkA and trkB receptors in astrocytes and plaques in Alzheimer's disease might be related to beta-amyloid deposition and could be implicated in the development of Alzheimer's disease.

Increased expression of type 1 insulin-like growth factor receptor messenger RNA in rat hippocampal formation is associated with aging and behavioral impairment

Insulin-like growth factor messenger RNAs are expressed in adult rat brain. However, little is known about the effects of aging on the expression of the insulin-like growth factors, their receptors, and their binding proteins in different regions of rat brain. The goal of the current study was to assess whether there is altered expression of the insulin-like growth factor system during normal aging in the hippocampal formation, a region particularly vulnerable to the aging process. A spatial learning task in the Morris water maze was used to assess the cognitive status of young (7-8-month-old) and aged (28-29-month-old) male Long-Evans rats. Sites of expression and abundance of insulin-like growth factor-I, type 1 insulin-like growth factor receptor, and insulin-like growth factor binding protein-4 messenger RNAs were then examined by in situ hybridization histochemistry and solution or northern blot hybridization assays. In situ hybridization histochemistry revealed no qualitative differences in the regional distribution of insulin-like growth factor-I, type 1 receptor, and insulin-like growth factor binding protein-4 messenger RNAs within the hippocampal formation of young and aged rats. However, quantitative analysis of messenger RNA abundance in hippocampal tissue homogenates showed a significant age-related increase in type 1 receptor messenger RNA (n = 25; t = -2.5; P < 0.02). Furthermore, linear regression analysis indicated that type 1 receptor messenger RNA abundance was significantly correlated with spatial learning impairment in the water maze (r = 0.44; P < 0.03) such that greater behavioral impairment was associated with higher type 1 receptor messenger RNA levels in the hippocampal formation. Neither insulin-like growth factor-I nor insulin-like growth factor binding protein-4 messenger RNA abundance was related to age or behavior. However, linear regression revealed a negative correlation between insulin-like growth factor-I messenger RNA abundance and type 1 receptor messenger RNA abundance in aged hippocampus (r = -0.72, P < 0.01). These data indicate that increased hippocampal expression of type 1 receptor messenger RNA is associated with aging and cognitive decline. The correlation between type 1 receptor and insulin-like growth factor-I messenger RNA abundance in the hippocampal formation of aged rats suggests that insulin-like growth factor availability may influence type 1 receptor expression. However, because no overall age difference was found in the amount of insulin-like growth factor-I messenger RNA in the hippocampal formation, decreased insulin-like growth factor from other sources such as the cerebrospinal fluid and the peripheral circulation may be involved in up-regulating type 1 receptor messenger RNA. Alternatively, type 1 receptor messenger RNA regulation may be part of a trophic response to the degenerative and regenerative events that occur within the hippocampal formation during aging.

Interleukin-6 released in human cerebrospinal fluid following traumatic brain injury may trigger nerve growth factor production in astrocytes

Cytokines are involved in nerve regeneration by modulating the synthesis of neurotrophic factors. The role played by interleukin-6 (IL-6) in promoting nerve growth factor (NGF) after brain injury was investigated by monitoring the release of IL-6 and NGF in ventricular cerebrospinal fluid (CSF) of 22 patients with severe traumatic brain injuries. IL-6 was found in the CSF of all individuals and remained elevated for the whole study period. NGF appeared in the CSF if IL-6 levels reached high concentrations and was often detected simultaneously with or following an IL-6 peak. The amounts of NGF correlated with the severity of the injury, as indicated by the clinical outcome of the patients. The functional relationship of IL-6 and NGF was investigated utilizing cultured mouse astrocytes. The CSF of 8 patients containing IL-6 induced NGF production in astrocytes, whereas control CSF without IL-6 had no effect. The induction of NGF was inhibited up to 100% by adding anti-IL-6 antibodies. These results were corroborated when astrocytes were exposed to recombinant IL-6 at different concentrations resulting in NGF production. Thus, the production of IL-6 within the injured brain may likely contribute to the release of neurotrophic factors by astrocytes.

Effects of intraventricular encapsulated hNGF-secreting fibroblasts in aged rats

Exogenous NGF administered into the central nervous system (CNS) has been reported to improve cognitive function in aged rats. However, concerns have been expressed about the risks involved with supplying NGF to the CNS. In this study, baby hamster kidney cells (BHK) genetically modified to secrete human NGF (hNGF) were encapsulated in semipermeable membranes and implanted intraventricularly. ChAT/LNGFR-positive basal forebrain neurons were shown to atrophy and degenerate with age, especially in cognitively impaired rats. The encapsulated BHK-NGF cells produced less than 10% of doses previously reported to be effective, but this was sufficient to increase the size of ChAT/LNGFR-positive basal forebrain neurons in the aged and learning-impaired rats to the size of the neurons in young healthy rats. The hNGF from these encapsulated cells also improved performance in a repeated-acquisition version of the Morris water maze spatial learning task in learning-impaired 20.6- and 26.7-mo-old rats. Furthermore, there was no evidence that these doses of hNGF impaired Morris water maze performance in the youngest 3.3-5.4 mo rats, and analyses of mortality rates, body weights, somatosensory thresholds, potential hyperalgesia, and activity levels, suggested that these levels of exogenous hNGF are not toxic or harmful to aged rats. These results suggest that CNS-implanted semipermeable membranes, containing genetically modified xenogeneic cells continuously producing these levels of hNGF, attenuate age-related cognitive deficits in nonimmunosuppressed aged rats, and that both the surgical implantation procedure and long-term exposure to low doses of hNGF appear safe in aged rats.

Dendritic reorganisation in the basal forebrain under degenerative conditions and its defects in Alzheimer's disease. II. Ageing, Korsakoff's disease, Parkinson's disease, and Alzheimer's disease

Changes in the dendritic arborisation of Golgi-impregnated basal forebrain neurones with respect to size, shape, orientation, and topology of branching were quantitatively investigated in ageing, Alzheimer's disease (AD), Korsakoff's disease (KD), and Parkinson's disease (PD). A reorganisation of the whole dendritic tree characterized by an increase in both the total dendritic length and the degree of dendritic arborisation as well as by changes in the shape of the dendritic field was found during ageing, in KD, PD, and AD. Dendritic growth under these conditions was related to the extent of cell loss in basal forebrain nuclei. There appeared to be major differences, however, with respect to the overall pattern of dendritic reorganisation between AD on one side and ageing, KD, and PD on the other side. In both ageing and KD, dendritic growth was largely restricted to the terminal dendritic segments, resulting in an increase of the size of the dendritic field (pattern of "extensive growth") In AD, however, dendritic growth mainly resulted in an increase of the dendritic density within the dendritic field without being accompanied by an increase in the size of the volume occupied by the dendritic tree (pattern of "intensive growth"). In AD, aberrant growth processes were frequently observed in the perisomatic area or on distal dendritic segments of basal forebrain neurones of the reticular type. Neurones with aberrant growth profiles were typically located in the direct vicinity of deposits of beta/A4 amyloid. Perisomatic growth profiles were covered by the low-affinity receptor of nerve growth factor p75NGFR. Aberrant growth processes were not present in ageing, KD, and PD. On the basis of the present study, it is concluded that under certain degenerative conditions, reticular basal forebrain neurones undergo a compensatory reorganisation of their dendritic arborisation, a process that has become defective in AD, thereby converting a physiological signal into a cascade of events contributing to the pathology of the disease.

Dendritic reorganisation in the basal forebrain under degenerative conditions and its defects in Alzheimer's disease. I. Dendritic organisation of the normal human basal forebrain

In the present study, the dendritic organisation of neurones in the normal human basal forebrain was analysed as a prerequisite for the evaluation of pathological changes occurring in Alzheimer's disease and related conditions (see other Arendt et al. papers in this issue). Neurones in the basal nucleus of Meynert (NbM), the nucleus of the vertical limb of the diagonal band, and the medial septal nucleus were examined after Golgi impregnation. Cells were classified according to the dendritic branching pattern and soma shape as either reticular neurones or multipolar giant neurones. The reticular type of neurones constitutes more than 90% of neurones in the BnM. Cholinergic neurones also belong to this cell type. Reticular neurones were further subdivided into four subtypes. Morphological features and arrangement of reticular basal forebrain neurones were identical to those described for "reticular formation cells" or "isodendritic" neurones. Dendritic trees of reticular neurones show a spatial orientation perpendicular to passing fibres as well as a high degree of overlap, both of which are hallmarks of "open nuclei." The qualitative classification of Golgi-impregnated basal forebrain neurones was substantiated by a computer-based three-dimensional analysis. Topologic and metric parameters of the dendritic tree were calculated for each type of neurone to characterise the degree of dendritic branching, the shape and orientation of the dendritic arborisation, the spatial extension of the dendritic tree, and soma size. The classification criteria were evaluated according to their power of discrimination between different cell types by means of a discriminant analysis. The quantitative approach applied in the present study not only provides an objective measure for the description and comparison of the structure of various types of neurones but also makes it possible to elucidate fine structural changes that might occur under pathologic conditions and that are not evident during qualitative studies alone.

Implants of polymer-encapsulated human NGF-secreting cells in the nonhuman primate: rescue and sprouting of degenerating cholinergic basal forebrain neurons

Baby hamster kidney (BHK) cells were genetically modified to secrete high levels of human nerve growth factor (BHK-hNGF). Following polymer encapsulation, these cells were implanted into the lateral ventricle of four cynomolgus monkeys immediately following a unilateral transection/aspiration of the fornix. Three control monkeys received identical implants, with the exception that the BHK cells were not genetically modified to secrete hNGF and thus differed only by the hNGF construct. One monkey received a fornix transection only. All monkeys displayed complete transections of the fornix as revealed by a comprehensive loss of acetylcholinesterase-containing fibers within the hippocampus ipsilateral to the lesion. Control monkeys that were either unimplanted or received BHK-control (non-NGF secreting) cell implants did not differ from each other and displayed extensive losses of choline acetyltransferase and p75 NGF receptor (NGFr)-immunoreactive neurons within the medial septum (MS; 53 and 54%, respectively) and vertical limb of the diagonal band (VLDB; 21 and 30%, respectively) ipsilateral to the lesion. In contrast, monkeys receiving implants of BHK-hNGF cells exhibited a only a modest loss of cholinergic neurons within the septum (19 and 20%, respectively) and VLDB (7%). Furthermore, only implants of hNGF-secreting cells induced a dense sprouting of cholinergic fibers within the septum, which ramified against the ependymal lining of the ventricle adjacent to the transplant site. Examination of the capsules retreived from monkeys just prior to their death revealed an abundance of cells that produced detectable levels of hNGF in a sufficient concentration to differentiate PC12A cells in culture. These findings support the use of polymer-encapsulated cell therapy as a potential treatment for neurodegenerative diseases such as Alzheimer disease where basal forebrain degeneration is a consistent pathological feature. Moreover, this encapsulated xenogeneic system may provide therapeutically effective levels of a number of neurotrophic factors, alone or in combination, to select populations of neurons within the central nervous system.

Enhanced neurotrophic activity in Alzheimer's disease cortex is not associated with down-regulation of metallothionein-III (GIF)

Alzheimer's disease (AD) is a chronic neurodegenerative disorder for which the pathogenic mechanisms are not well understood. Previous studies demonstrated that extracts prepared from AD brains could increase the survival of rat cortical neurons in vitro. Additional studies indicated that this enhanced neurotrophic activity of AD brain was due to a reduction of a growth inhibitory factor (GIF) that was subsequently shown to be a new member of the metallothionein (MT) gene family, and designated MT-III. The study presented here examined the association between neurotrophic activity and MT-III expression in frontal cortices from eight AD and five control brains, and further characterized the inhibitory activity of MT-III. On average, AD extracts stimulated the survival of approximately 2-fold more rat cortical neurons than control extracts, demonstrating that AD brain possesses elevated neurotrophic activity. When recombinant MTs were added to cultures grown in the presence of brain extract, MT-III but not MT-I had an inhibitory effect on neuron survival, confirming that MT-III is a specific inhibitory factor in this assay. However, in contrast to previous reports, neither MT-III mRNA nor MT-III protein levels were significantly decreased in the AD group. Therefore, the difference in neurotrophic activity between the AD and control brain samples examined in this study is probably not directly mediated by MT-III. These results suggest that MT-III down-regulation is not an important pathogenic event in some cases of AD.

Functional consequences of a single nerve growth factor administration following septal damage in rats

This study examined how possible nerve growth factor (NGF)-induced behaviour changes after septal damage might be modulated by the lesion extent, the dose of NGF administered and the delay between surgery and the onset of testing. In a first experiment, young rats which received electrolytic septal lesions of high or low intensity (inducing respectively large and mild lesions) were treated with 10 or 30 micrograms NGF administered intrahippocampally in a single injection. They were tested 4 months postoperatively for open field ambulation, spontaneous alternation and radial maze performance. It was observed that irrespective of the severity of the lesions rats were impaired in the spontaneous alternation and radial maze tests; however, no obvious changes appeared in the open field test. While an NGF injection did not affect behavioural performances in rats with large lesions, it was capable of ameliorating behavioural deficits in the spontaneous alternation and radial maze tests of rats with mild lesions in both NGF dosage groups. It was also seen that lesions produced a general decrease in hippocampal choline acetyltransferase (ChAT) activity, which was not significantly affected by an NGF administration. There was no significant correlation between ChAT activity and behavioural performance of NGF-treated rats. In a second experiment, young rats received mild septal lesions and were treated with 10 micrograms NGF. These rats were tested 2 weeks postoperatively for radial maze performance. NGF rats exhibited similar behaviour to controls with regard to all of the variables measured. The present results suggest that a single NGF administration spares some abilities to use spatial information efficiently providing lesions are partial.

An analysis of the effects of Alzheimer's plaques on living neurons

Although senile plaques represent a consistent neuropathological feature in Alzheimer's brains, it is not known what role plaques play in the etiology of the disease. Both growth-promoting and growth-inhibiting influences have been postulated. One of the major components in plaques, beta-amyloid, has been shown to affect neuron survival and neurite outgrowth in vitro. Because plaques consist of other components in addition to beta-amyloid, we undertook the present study to determine whether neuronal survival and neurite outgrowth are affected by the presence of a senile plaque. This was accomplished by using cryostat sections from the cerebral cortex of Alzheimer's patients as a substratum for cultured rat hippocampal neurons. Evaluation of these living neurons on Alzheimer's tissue demonstrated that senile plaques affect the amount, complexity, and direction of neurite outgrowth. In addition, neurons were more likely to extend processes away from plaques rather than toward a plaque. Although cell survival on plaques and in control regions was similar, cell survival was significantly reduced in the peri-plaque region. These observations suggest that senile plaques could have deleterious effects on neural organization in situ.

CNS glial cells express neurotrophin receptors whose levels are regulated by NGF

Normal CNS glial cells manufacture neurotrophin receptors and are competent to respond to NGF. Neurotrophins bind a common receptor (LNGFR) and ligand-specific, tyrosine kinase-containing subunits (TrkA, TrkB, or TrkC). Northern blots and transcription assays reveal complex transcriptional regulation of LNGFR in astrocytes; from undetectable basal levels, NGF dramatically induces LNGFR within 4-6 h. Oligodendrocytes' relatively high basal levels are unaffected by NGF. TrkA mRNA was undetectable, however, TrkB was present and upregulated by NGF in astrocytes but not oligodendrocytes. The results are consistent with receptor autoregulation by its ligand and suggest that NGF plays a role in normal glial functions.

Perisomatic sprouts immunoreactive for nerve growth factor receptor and neurofibrillary degeneration affect different neuronal populations in the basal nucleus in patients with Alzheimer's disease

Aberrant morphological profiles, most likely reflecting a process of perisomatic sprouting, have been detected in Alzheimer's disease (AD) on neurons of the basal nucleus of Meynert by means of Golgi impregnation. Using the mouse monoclonal antibody, ME 20.4., perisomatic profiles were shown to be immunoreactive for nerve growth factor receptor (NGFR). Double label studies using either thioflavin S or anti-neurofibrillary tangle (NFT) antibody B5 in combination with ME 20.4 demonstrated that neurons with aberrant growth profiles failed to express neurofibrillary tangle-bearing material, which otherwise could be detected in large amounts throughout the basal forebrain. The results indicate that in AD, neurons in the basal forebrain, not affected by neurofibrillary degeneration, respond to an increased trophic influence. Dendritic sprouting in AD might, therefore, more likely be regarded as an attempt by the nervous system to repair itself following damage rather than as an abnormal process with primary pathologic significance leading to cellular degeneration.

Human serum stimulates Alzheimer markers in cultured hippocampal neurons

The mechanism for promoting the distinct types of lesions in the Alzheimer disease (AD) brain and other changes outside the brain is unknown. We examined neurons in culture, unprotected by glia or a blood-brain barrier, to determine if exposure to serum from Alzheimer patients would affect markers for Alzheimer brain lesions. Rat hippocampal neurons were first grown for 4 days in a new serum-free culture medium, then exposed for 24 hr to human sera. Sera from 12 AD patients or their spouses increased four molecular markers characteristic of Alzheimer senile plaques and neurofibrillary tangles: Alz-50, beta-amyloid (beta/A4), MAP2, and ubiquitin, each with their expected cytologic distributions. Sera from ten young adults produced significantly less stimulation. By quantitative immunofluorescence, neuronal exposure to the elderly human sera produced 1.8- to 2.5-fold increases in mean fluorescent area/cell for each of these four markers relative to no serum exposure. As controls, an unrelated neuronal marker, enolase, was unaffected and fetal bovine serum did not stimulate immunoreactivity. Neuron viability and somal area were unaffected at 24 hours. The MAP2 increases were dose dependent with negligible effect at 2% serum and maximum effect at 10% serum after 24 hr. The MAP2 increase was greater after 48 hr of exposure than 24 hr and negligible at 2 hr. This stimulation of AD markers by human serum suggests that the genesis of both neuronal plaques and tangles may arise from access of toxic serum factors to susceptible neurons and/or failure to detoxify these factors.

Stability of septohippocampal neurons following excitotoxic lesions of the rat hippocampus

The present study examined the effects of removing hippocampal nerve growth factor (NGF)-producing neurons upon cholinergic and noncholinergic septohippocampal projecting neurons. To deplete septal/diagonal band neurons of their intrinsic source of NGF, rats received unilateral intrahippocampal injections of ibotenic acid and were sacrificed 2-24 weeks later. Choline acetyltransferase and parvalbumin immunohistochemistry failed to reveal changes in the number of cholinergic or gamma-aminobutyric acid-containing neurons, respectively, within the septal/diagonal band region ipsilateral to the hippocampal lesion at any time point examined. Additionally, immunocytochemical localization of nonphosphorylated and phosphorylated neurofilament proteins did not reveal abnormal staining characteristics within the septal/diagonal band complex, suggesting that this lesion does not alter cytoskeletal features of neurons which project to the hippocampus. Selected rats received unilateral hippocampal lesions and 3 months later were injected with fluorogold into the remaining hippocampal remnant and with wheat germ agglutinin conjugated to horse radish peroxidase into the intact contralateral hippocampus. Both retrograde tracers were predominantly transported to their respective ipsilateral septum and vertical limb of the diagonal band. This indicates that following the lesion, septal/diagonal band neurons still project ipsilaterally and sprouting to the NGF-rich contralateral side does not occur. RNA blot analysis revealed a decrease in NGF mRNA expression within the lesioned hippocampus with a maximum reduction of approximately 70%. In contrast, no change in NGF mRNA expression was observed within the ipsilateral septum relative to the contralateral side. The present study demonstrates that removal of hippocampal target neurons does not alter the number, morphology, or projections of both cholinergic and noncholinergic septal/diagonal band neurons.

Serotonergic fibres degenerating in the aging rat brain or sprouting from grafted fetal neurons are not affected by the neurotrophic ACTH analogue Org 2766

The effects of chronic treatment with the purported neurotrophic factor ACTH(4-9) analogue Org 2766 were studied on age-related degeneration of serotonergic fibres and on gliosis in the rat hippocampus and caudate putamen complex. In addition, the potential growth-promoting effects of Org 2766 were investigated on fetal serotonergic cells implanted in a previously denervated hippocampus of young adult rats. Chronic treatment of rats from the age of 11 months to 17-18 months did not affect the incidence of aberrant serotonergic fibres in the caudate-putamen complex or the fibres densities in the hippocampus or the caudate-putamen complex. Gliosis was unaffected by Org 2766 treatment as indicated by increased number and staining intensity of glial fibrillary acidic protein-immunoreactive cell bodies in both brain areas. Grafting of fetal raphe cells in young adult rats caused a time-dependent reinnervation of the previously denervated hippocampus. The reinnervation was not affected by treatment of the rats with Org 2766 for 4 weeks following implantation.

Nerve growth factor and septal grafts: a study of behavioral recovery following partial damage to the septum in rats

Previous studies have produced conflicting results about the effects of intracerebral injection of NGF after septal damage in rats: in one experiment, behavioral deficits in maze tasks were exacerbated by NGF administration whereas they were alleviated in another one. The present investigation aimed to clarify the effects of NGF and to identify factors liable to induce different behavioral outcomes. Behavioral effects were assessed following a postsurgical delay of five months using various parameters: food consumption in a novel environment, spontaneous activity, locomotion in an open-field, immobility in a tail suspension test, spontaneous alternation in a T-maze and performance in a radial eight-arm maze. Possible influence of intrahippocampal sympathetic fiber ingrowth occurring after septal lesions was ruled out, as the comparison of rats subjected to superior cervical ganglia removal with their lesion-control counterparts showed few behavioral differences, even after NGF administration. All lesioned rats showed reduced adaptability in most of these tests. Grafts partially reversed the lesion-induced deficit in spontaneous alternation. A single intracerebral NGF injection was found to ameliorate radial maze performance, whether rats were grafted or not. However, it appeared that the number of strategies available to NGF-rats in the radial maze task was as limited as for lesion-control rats. These findings suggest that NGF-rats do not recover spatial abilities lost after septal lesions, but are able to make more efficient use of remaining capacities to master the maze task.

Exogenous nerve growth factor reverses age-related structural changes in neocortical neurons in the aging rat. A quantitative Golgi study

The role of chronic exogenous intracerebroventricular administration of nerve growth factor (NGF) on the morphology of layer V pyramidal cell dendrites in aging rats was quantified using Golgi impregnations. Both dendritic branching and dendritic spines from the basilar tree of randomly selected pyramidal neurons of the frontal cortex were evaluated in young control (4-month-old) Fischer 344 rats, in old controls (24-month-old), and in 24-month-old rats administered NGF for 4 weeks. Sholl analysis of basilar dendritic trees showed that neuronal branching in older rats was significantly greater than that in young rats (probably due to compensatory dendritic hypertrophy). The extent of dendritic material in aged rats receiving NGF, however, was identical to that in young rats, that is, the dendritic tree had regressed in size. Dendritic spine response to NGF treatment depended on the region of the dendritic tree sampled. Normal aging resulted in spine loss. However, NGF treatment restored dendritic spine densities to those seen in young controls on terminal tip segments ("plastic" regions). Internal branch segments ("nonplastic" regions) showed no response to NGF. As dendritic spines are thought to represent the neuroanatomic basis of learning and memory, results suggest that NGF can influence the morphology of cortical neurons (probably indirectly via the basal forebrain projections) and therefore may play an efficacious role in the treatment of geriatric cognitive dysfunction and even perhaps in Alzheimer's disease.

Neurite outgrowth on postmortem human brain cryostat sections: studies of non-Alzheimer's and Alzheimer's tissue

An in vitro assay to test for regional differences in neurite growth-promoting and growth-inhibiting factors in tissue sections of CNS tissue has been adapted to the use of postmortem human brain tissue. Frozen sections of the temporal lobe from victims of Alzheimer's disease were used as substrates for sympathetic neurite outgrowth in tissue culture. Tissue sections from a non-Alzheimer's brain were used as a control. Both explanted chick sympathetic ganglia and dissociated chick sympathetic neurons were cultured for 3 to 5 days on tissue sections in the presence of exogenous nerve growth factor. The dichotomy between gray and white matter portions of the tissue sections in supporting neurite outgrowth that was previously reported for fresh frozen human brain tissue was also found to persist in postmortem tissue. In addition, the total neurite outgrowth from explanted sympathetic ganglia was found to be significantly less on postmortem sections when compared with previous results obtained from fresh frozen tissue samples of epileptic tissue. Dissociated neurons exhibited neurite outgrowth on Alzheimer's sections that showed preferential growth on blood vessel segments but no affinity for senile plaques. The results suggest that there is some decline in the neurite growth-promoting ability of cortical gray matter obtained from postmortem-derived brains when compared with fresh tissue and that senile plaques do not represent sites of neurite stimulation in this in vitro system.

Recombinant human nerve growth factor infusions prevent cholinergic neuronal degeneration in the adult primate brain

Atrophy of cholinergic neurons is a prominent component of Alzheimer's disease, and may explain in part the profound memory loss that is characteristic of patients with this disorder. Previous studies in animal models have shown that infusions of nerve growth factor into the adult brain can prevent both age-related and lesion-induced cholinergic neuronal atrophy. Recently, recombinant human nerve growth factor was found biologically active in nonprimate animal models. In the present experiment, recombinant human nerve growth factor infusions into the brains of adult primates prevented lesion-induced cholinergic neuronal degeneration and promoted cholinergic neurite sprouting. These findings provide additional support for potential therapeutic trials of human nerve growth factor in patients with Alzheimer's disease.

The growth inhibitory factor that is deficient in the Alzheimer's disease brain is a 68 amino acid metallothionein-like protein

We have purified and characterized the growth inhibitory factor (GIF) that is abundant in the normal human brain, but greatly reduced in the Alzheimer's disease (AD) brain. GIF inhibited survival and neurite formation of cortical neurons in vitro. Purified GIF is a 68 amino acid small protein, and its amino acid sequence is 70% identical to that of human metallothionein II with a 1 amino acid insert and a unique 6 amino acid insert in the NH2-terminal and the COOH-terminal portions, respectively. The antibodies to the unique sequence of GIF revealed a distinct subset of astrocytes in the gray matter that appears to be closely associated with neuronal perikarya and dendrites. In the AD cortex, the number of GIF-positive astrocytes was drastically reduced, suggesting that GIF is down-regulated in the subset of astrocytes during AD.

Activities in cellular signalling pathways: a two-edged sword?

The maintenance and adaptive plasticity of neural circuitry requires the coordinated activities of an array of signal transduction systems. These different signalling systems play critical roles in carving out and modifying functional neural circuitry in development and adult plasticity. Ironically, aberrant activity in these systems may sever neural connections and take the lives of neurons in a variety of disorders including Alzheimer's disease. A rather complex set of circumstances must be taken into account when considering the consequences of activity in pathways that regulate neuroarchitecture. The specific signalling pathways activated, their levels of activation, activity in other pathways, and environmental conditions are among the factors that enter into the equation for determining whether use is a boon or a bane.

Can activity modulate the susceptibility of neurons to degeneration?

The provocative suggestion that maintaining an appropriate level of continuous neuronal activation, either by training or pharmacological intervention, might make neurons more robust and able to withstand degenerative insults during aging is fascinating and clearly warrants further investigation. Two cautionary points: 1) it is far from certain that Alzheimer's disease represents accelerated aging, and 2) excessive activation could be harmful to neurons in a context other than that of normally occurring "wear and tear."

Plasticity in Alzheimer's disease: too much or not enough?

The goal of optimizing restorative sprouting in Alzheimer's disease is based on the premise that sprouting is beneficial and is deficient in AD. The beneficial aspects of neuronal plasticity have been questioned, however, and other studies suggest that some aspects of sprouting may be exaggerated in AD and contribute to the formation of plaques, tangles, and other neuropathological hallmarks of this disorder. Manipulation of the sprouting response may represent a promising treatment strategy in AD, but whether the goal is to augment or impede sprouting may depend upon the extent of the damage and the severity of the disease state.

Structural plasticity of synapses in Alzheimer's disease

Plasticity of the synaptic contact zone was previously observed following loss of synapses in the cerebral cortex of normal aging humans. The present study was undertaken to determine if there was quantitative evidence of synapse loss and synapse plasticity in the inferior temporal, superior parietal, parieto-occipital, and superior frontal cortical regions in Alzheimer's disease (AD), and how such changes related to the neurofibrillary tangles and amyloid plaques. The results showed that age at autopsy did not correlate with the numbers of synapses, plaques, or tangles. However, the numbers of synapses strongly reflected the pathology of AD; in all four brain regions, there were fewer synapses as the numbers of plaques and tangles increased. In the inferior temporal and superior parietal cortices, the loss of synapses was accompanied by an increase in the synaptic contact length. The results suggest that, in some cerebral cortical brain regions, synapses are capable of plasticity changes, even when the pathology of AD and loss of synapses are severe.

Thyroid hormone modulates the development of cholinergic terminal fields in the rat forebrain: relation to nerve growth factor receptor

Hyperthyroidism, induced in rat pups by the daily intraperitoneal administration of 1 microgram/g body weight triiodothyronine, facilitated the development of ChAT fiber plexuses in brain regions innervated by basal forebrain cholinergic neurons, leading to an earlier and increased expression of cholinergic markers in those fibers in the cortex, hippocampus and amygdala. A similar enhancement was seen in the caudate-putamen complex. This histochemical profile was correlated with an accelerated appearance of ChAT-positive telencephalic puncta, as well as with a larger total number of cholinergic terminals expressed, which persisted throughout the eight postnatal week, the longest time examined in the present study. Hypothyroidism was produced in rat pups by adding 0.5% propylthiouracil to the dams' diet beginning the day after birth. This dietary manipulation resulted in the diminished expression of ChAT in forebrain fibers and terminals. Hypothyroid treatment also reduced the quantity of ChAT puncta present during postnatal weeks 2 and 3, and, from week 4 and continuing through week 6, the number of ChAT-positive terminals in the telencephalic regions examined was actually less than the amount extant during the former developmental epoch. Immunostaining for nerve growth factor receptor (NGF-R), which is associated almost exclusively with ChAT-positive somata and fibers in the basal forebrain, demonstrated a different time course of postnatal development. Forebrain fibers and terminals demonstrating NGF-R were maximally visualized 1 week postnatally, a time at which these same neuronal elements evinced minimal ChAT-like immunopositivity. Thereafter and correlated with increased immunoreactivity for ChAT, fine details of NGF-R stained fibers were observed less frequently. Although propylthiouracil administration decreased NGF-R immunodensity, no alteration in the development of that receptor was observed as a function of triiodothyronine treatment. Cholinergic terminals in the ventrobasal thalamus, which derive from ChAT-positive neurons in the pedunculopontine and laterodorsal tegmental nucleus, were unaffected by either hyperthyroid or hypothyroid conditions. These cells also do not demonstrate NGF-R. We conclude from these experiments (1) that cholinergic fiber plexuses eventually exhibiting ChAT positivity in the telencephalon demonstrate NGF-R prior to the cholinergic synthetic enzyme, (2) that susceptibility to thyroid hormone manipulations may involve sensitivity to NGF, at least in some forebrain cholinergic systems and (3) that the effects of thyroid hormone imbalances on brain cholinergic neurons are regionally selective.