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

Metformin and its sulphonamide derivative simultaneously potentiateanti-cholinesterase activity of donepezil and inhibit beta-amyloid aggregation

The aim of this study was to assess in vitro the effects of sulphenamide and sulphonamide derivatives of metformin on the activity of human acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), establish the type of inhibition, and assess the potential synergism between biguanides and donepezil towards both cholinesterases (ChEs) and the effects on the β-amyloid aggregation. Sulphonamide 5 with para-trifluoromethyl- and ortho-nitro substituents in aromatic ring inhibited AChE in a mixed-type manner at micromolar concentrations (IC50 = 212.5 ± 48.3 µmol/L). The binary mixtures of donepezil and biguanides produce an anti-AChE effect, which was greater than either compound had alone. A combination of donepezil and sulphonamide 5 improved the IC50 value by 170 times. Compound 5 at 200 µmol/L inhibited Aβ aggregation by ∼20%. In conclusion, para-trifluoromethyl-ortho-nitro-benzenesulphonamide presents highly beneficial anti-AChE and anti-Aβ aggregation properties which could serve as a promising starting point for the design and development of novel biguanide-based candidates for AD treatment.

Metformin - a Future Therapy for Neurodegenerative Diseases : Theme: Drug Discovery, Development and Delivery in Alzheimer's Disease Guest Editor: Davide Brambilla

Type 2 diabetes mellitus (T2DM) is a complex, chronic and progressive metabolic disease, which is characterized by relative insulin deficiency, insulin resistance, and high glucose levels in blood. Esteemed published articles and epidemiological data exhibit an increased risk of developing Alzheimer's disease (AD) in diabetic pateints. Metformin is the most frequently used oral anti-diabetic drug, which apart from hypoglycaemic activity, improves serum lipid profiles, positively influences the process of haemostasis, and possesses anti-inflammatory properties. Recently, scientists have put their efforts in establishing metformin's role in the treatment of neurodegenerative diseases, such as AD, amnestic mild cognitive impairment and Parkinson's disease. Results of several clinical studies confirm that long term use of metformin in diabetic patients contributes to better cognitive function, compared to participants using other anti-diabetic drugs. The exact mechanism of metformin's advantageous activity in AD is not fully understood, but scientists claim that activation of AMPK-dependent pathways in human neural stem cells might be responsible for the neuroprotective activity of metformin. Metformin was also found to markedly decease Beta-secretase 1 (BACE1) protein expression and activity in cell culture models and in vivo, thereby reducing BACE1 cleavage products and the production of Aβ (β-amyloid). Furthermore, there is also some evidence that metformin decreases the activity of acetylcholinesterase (AChE), which is responsible for the degradation of acetylcholine (Ach), a neurotransmitter involved in the process of learning and memory. In regard to the beneficial effects of metformin, its anti-inflammatory and anti-oxidative properties cannot be omitted. Numerous in vitro and in vivo studies have confirmed that metformin ameliorates oxidative damage.

Selective cell death of hyperploid neurons in Alzheimer's disease

Aneuploidy, an abnormal number of copies of a genomic region, might be a significant source for neuronal complexity, intercellular diversity, and evolution. Genomic instability associated with aneuploidy, however, can also lead to developmental abnormalities and decreased cellular fitness. Here we show that neurons with a more-than-diploid content of DNA are increased in preclinical stages of Alzheimer's disease (AD) and are selectively affected by cell death during progression of the disease. Present findings show that neuronal hyperploidy in AD is associated with a decreased viability. Hyperploidy of neurons thus represents a direct molecular signature of cells prone to death in AD and indicates that a failure of neuronal differentiation is a critical pathogenetic event in AD.

Alzheimer's disease as a disorder of dynamic brain self-organization

Mental function is based on the dynamic organization of neuronal networks. In particular, phylogenetically young brain areas (e.g., cortical associative circuits), involved in the realization of "higher brain functions" such as learning, memory, perception, self-awareness, and consciousness, are continuously re-adjusted even after development is completed. By this life-long self-optimization process, epigenetic information remodels the cognitive, behavioral and emotional reactivity of an individual to meet the environmental demands. To organize brain structures of increasing complexity during evolution, the process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. The mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodeling according to experience, are accompanied, however, by an increasing inherent potential of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time may provide the basis for selective neuronal vulnerability. The mechanisms of synaptic plasticity, i.e., of modifiable interneuronal connectivity, are largely based on external morphoregulatory cues and internal signaling pathways that nonneuronal cells have phylogenetically acquired to sense their relationship to the local neighborhood and to control proliferation and differentiation in the process of tissue repair and regeneration after development is completed. Differentiated neurons that have withdrawn from the cell cycle use these molecular machinery alternatively to control synaptic plasticity. The existence of these alternative effector pathways within a neuron puts it on the risk to erroneously convert signals derived from plastic synaptic changes into positional cues that will activate the cell cycle. This cell cycle activation potentially links synaptic plasticity to cell death. Preventing cell cycle activation by locking neurons in a differentiated but still highly plastic phenotype will, thus, be crucial to prevent neurodegeneration.

Neurodegeneration and plasticity

Neurofibrillary degeneration, associated with the formation of paired helical filaments (PHF), is one of the critical neuropathological hallmarks of Alzheimer's disease (AD). Although the microtubule-associated protein tau in a hyperphosphorylated form has been established as primary PHF constituent, the process of tau phosphorylation and its potential link to degeneration is not very well understood, mostly because of the lack of a physiological in vivo model of PHF-like tau phosphorylation. PHF formation in AD follows a hierarchical pattern of development throughout different cortical areas, which closely matches the pattern of neuronal plasticity in the adult brain. Those brain areas are most early and most severely affected which are involved in the regulation of memory, learning, perception, self-awareness, consciousness, and higher brain functions that require a life-long re-fitting of connectivity, a process based on a particularly high degree of plasticity. Failures of synaptic plasticity are, thus, assumed to represent early events in the course of AD that eventually lead to alteration of tau phosphorylation. Recently, we have used the hibernation cycle, a physiological model of adaptation associated with an extraordinary high degree of structural neuronal plasticity, to analyze the potential link between synaptic plasticity, synaptic detachment and the regulation of tau phosphorylation. During torpor, a natural state of hypothermia, synaptic contacts between mossy fibers and hippocampal pyramidal neurons undergo dramatic regressive changes that are fully reversible very rapidly during euthermy. This rapid, reversible, and repeated regression of synaptic and dendritic components on CA3 neurons is associated with a reversible PHF-like phosphorylation of tau at a similar time course. The repeated formation and degradation of PHF-tau might, thus, represent a physiological mechanism not necessarily associated with pathological effects. These findings implicate an essential link between neuronal plasticity and PHF-like phosphorylation of tau, potentially involved in neurofibrillary degeneration.

Activation of mitogen-activated protein kinase cascade and phosphorylation of cytoskeletal proteins after neurone-specific activation of p21ras. I. Mitogen-activated protein kinase cascade

Alterations in the phosphorylation state of the microtubule-associated protein tau have been associated with the pathogenesis of neurofibrillary degeneration as well as with a neuroprotective action against apoptotic cell death. Mitogen-activated protein kinases (MAPK) phosphorylate tau protein in vitro but the pathophysiological significance of this tau phosphorylation and its effects on neuronal viability is far from clear. Moreover, an in vivo model of activation of MAPK, a key candidate for in vivo tau phosphorylation, is still lacking. The aim of the present study and the accompanying paper was to establish an animal model of stimulated MAPK and to analyse the consequences on tau phosphorylation and the neuronal cytoskeleton. We took advantage of transgenic mice with neurone-specific expression of activated ras protein (p21H-ras(Val12)). The expression of the transgene in these animals is forced to a subset of neurones by the use of the synapsin I promoter. Activity of B-raf was elevated by 37%, while activity of MAPK (ERK1/ERK2) was increased by 25% associated with a subcellular redistribution from the cytoplasmic to the nuclear compartment. Kinases downstream of MAPK such as p90rsk and glycogen synthase kinase 3beta were only marginally affected. Activity of p70S6 kinase was unaltered. The present model might be useful to study the effects of activation of the MAPK cascade on tau phosphorylation and its cell biological sequelae.

Disturbance of neuronal plasticity is a critical pathogenetic event in Alzheimer's disease

Brain areas affected by AD pathology are primarily those structures that are invovled in the regulation of "higher brain functions". The functions these areas subserve such as learning, memory, perception, self-awareness, and consciousness require a life-long re-fittng of synaptic contacts that allows for the acquistion of new epigenetic information, a process based on a particularly high degree of structural plasticity. Here, we outline a hypothesis that it is the "labile state fo differentiation" of a subset of neurons in the adult brain that allows for ongoing neuroplastic processes after development is completed but at the same time renders these neurons particularly vulnerable. Mechanisms of molecular and cellular control of neuronal differentiation and proliferation might, thus, not only play a role during development but critically involved in the pathogenesis of neurodegeneration.

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".

Aberrancies in signal transduction and cell cycle related events in Alzheimer's disease

Neurodegeneration in Alzheimer's disease (AD) is associated with the appearance of dystrophic neuronal growth profiles that most likely reflects an aberrant attempt of neuronal repair. This process of neuronal reorganisation, which eventually goes awry and becomes a disease itself, might be initiated physiologically as a response to neuronal injuries. Minor neuronal damage due to a variety of life events or genetic pertubations that are usually compensated in the normal adult brain by adaptation and repair might thus be amplified and accumulated, thereby resulting in a progressive neurodegeneration. The present paper summarizes recent evidence supporting the hypothesis that a primary impairment of intracellular signal transduction that is mediated by a hierarchy of phosphorylation signals and associated with a aborted attempt of neurons to re-enter the cell-cycle is a key element in the pathomechanism of AD. These changes might result in malfunction of neuronal adaptation and repair and eventually lead to neuronal death. During the process of aging as well as in chronic neurodegenerative disorders such as Alzheimer's disease (AD), the continuous but rather slow action of pathogenetic factors might give room for the activation of compensatory mechanisms, serving to regain a neuronal population much of its synaptic connectivity in the presence of cell loss. An upregulation in the capacity to synthesize and store neurotransmitters (Lapchak et al., 1991), an increased expression of trophic factors (Hellweg et al., 1990; Arendt et al., 1995a,b), as well as regenerative sprouting (Fritschy and Grzanna, 1992), synaptic enlargement (Scheff et al., 1990; Lippa et al., 1992), and neosynaptogenesis (Ramirez and Ulfhake, 1992) have been described among other processes.

Neuronal supernumerary and dendritic sprouting of the nucleus ambiguus after chronic alteration of peripheral targets in cats

Anatomic changes of neuronal profiles in response to chronic alteration of peripheral targets were investigated in the nucleus ambiguus (NA) of cats. Unilateral vagal-hypoglossal nerve anastomosis was performed by suturing the transected proximal stump of the vagus nerve to the transected distal stump of the hypoglossal nerve. After comparing horseradish peroxidase (HRP)-labeled neurons on the ipsilateral operated side of the NA with the contralateral unoperated NA and the NA following transection and reuniting to the vagus itself, a remarkable ramification and elongation of the dendritic trees was observed in the HRP-positive neurons on the ipsilateral NA. Quantitative analysis of neuronal profiles revealed that the number of the medium and large neurons on the ipsilateral NA was greater than the contralateral NA and the NA following autologous suturing of the vagus. Comparisons of variable dendritic lengths of the medium and large neurons on the ipsilateral NA revealed longer distances and more branches of the tertiary and perisomatic dendrites than those of the contralateral NA and the NA ipsilateral to autologous reunion. Our results suggest that remarkable sprouting and elongation of the dendritic trees as well as cell supernumerary occurred in the dominant NA motoneurons ipsilateral to the nerve anastomosis. In conclusion, there is a trophic influence in the tongue musculature, which was retrogradely transported to the NA neurons via the regenerating axons and caused the morphological changes in the NA in response to the rerouting of efferents from the vagus nerve to the hypoglossal nerve to innervate intimate tongue musculature.

Developmental regulation and PKC dependence of Alzheimer's-type tau phosphorylations in cultured fetal rat hippocampal neurons

Attempts to describe a mechanism of neurofibrillary tangle formation often focus on site specific phosphorylations of tau protein. These have typically been described in both Alzheimer's disease and developing brains. Therefore, study of the developmental regulation of Alzheimer epitope tau phosphorylations may help explain their persistence or recurrence during Alzheimer's disease. Using fetal rat hippocampal cultures, we report a spatial and temporal expression of tau phosphorylation during neuronal differentiation. We have examined phosphorylation at the epitopes recognized by monoclonal antibodies, PHF-1 and Tau 1. Tau was highly phosphorylated at the PHF-1 epitope at all culture ages examined using both immunohistochemical staining and Western blots. Tau was heavily phosphorylated at the Tau 1 epitope only in older cultures. The populations of tau recognized by the two antibodies also exhibited different solubilities, suggesting different microtubule binding behaviors: tau phosphorylated at PHF-1 was retained in axons following solubilization whereas Tau 1 immunoreactive tau was not retained in any cell compartment. Finally, in this culture system, maintenance of phosphorylation at the PHF-1 epitope, but not the Tau 1 epitope, required protein kinase C activity. These results indicate unique regulatory mechanisms and roles for each of these phosphorylated tau epitopes.

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.

Increased expression and subcellular translocation of the mitogen activated protein kinase kinase and mitogen-activated protein kinase in Alzheimer's disease

The sequential activation of the mitogen-activated protein kinase kinase and its substrate, the mitogen-activated protein kinase is involved in a cascade of protein kinases which link a number of cell surface signals to intracellular changes in enzyme activity and gene expression. In vitro, mitogen-activated protein kinase is able to phosphorylate the microtubule-associated protein tau at Ser-Pro and Thr-Pro sites, thereby generating abnormally hyperphosphorylated tau species that are similar to paired helical filament-tau found in Alzheimer's disease. In the present study, we analysed the levels of immunoreactive mitogen-activated protein kinase kinase and mitogen-activated protein kinase in the temporal cortex (area 22) of patients with Alzheimer's disease by means of enzyme-linked immuno-sorbent assays and compared these changes with the content of abnormally phosphorylated paired helical filament-tau. The levels of immunochemically detected mitogen-activated protein kinase kinase and mitogen-activated protein kinase were both increased in Alzheimer's disease by between 35 and 40% compared with age-matched controls. Elevation of mitogen-activated protein kinase kinase was most pronounced during early stages of Alzheimer's disease and was inversely related to the tissue content of abnormally phosphorylated paired helical filament-tau. Pronounced immunoreactivity of mitogen-activated protein kinase kinase and mitogen-activated protein kinase was present in both tangle bearing neurons and unaffected neurons of the temporal cortex. Immunoreactive neurons were most often localized in the direct vicinity of neuritic plaques. In Alzheimer's disease, the subcellular distribution of mitogen-activated protein kinase kinase and mitogen-activated protein kinase showed a striking translocation from the cytoplasmic to the nuclear compartment. It is suggested that the activation of the mitogen-activated protein kinase cascade which appears to be an early feature of Alzheimer's disease might be critically involved in self-stimulating processes of neurodegeneration and aberrant repair under these conditions.

Degeneration of rat cholinergic basal forebrain neurons and reactive changes in nerve growth factor expression after chronic neurotoxic injury--II. Reactive expression of the nerve growth factor gene in astrocytes

Long-term consumption of ethanol both in human and rodent induces a process of chronic degeneration of cholinergic basal forebrain neurons which results in a cholinergic deafferentation of the cortical mantle. We have used quantitative northern blot analysis and in situ hybridization to demonstrate that these degenerative events in rat evoke an increase in the expression of the nerve growth factor gene in a number of brain areas, including the cholinergic basal forebrain nuclei and their cortical target regions. By combining non-radioactive in situ hybridization and immunohistochemistry activated astrocytes were identified as the major source of altered nerve growth factor gene expression. This increased nerve growth factor expression is paralleled by a dendritic remodelling of basal forebrain neurons, while the expression of choline acetyltransferase in surviving neurons remains the same. This failure of nerve growth factor to rescue the expression of choline acetyltransferase differs from the effects of exogenously administered nerve growth factor in acutely lesioned systems. The results indicate that under certain conditions of chronic neurodegeneration, the utilization of nerve growth factor might be impaired, which could be due to a defective nerve growth factor signalling mechanism.

Degeneration of rat cholinergic basal forebrain neurons and reactive changes in nerve growth factor expression after chronic neurotoxic injury--I. Degeneration and plastic response of basal forebrain neurons

The process of degeneration and dendritic reorganization of cholinergic neurons was investigated in the rat basal forebrain under the conditions of chronic neurotoxic injury induced by long-term consumption of ethanol. After 28 weeks of ethanol treatment (20% v/v), both the number of choline acetyltransferase-immunoreactive basal forebrain neurons and levels of biochemical measures of cholinergic neurons, such as the activity of choline acetyltransferase and the synthesis and content of acetylcholine, were decreased by about 60-80%. The number of cholinergic neurons showing a positive hybridization signal to choline acetyltransferase messenger RNA was decreased to a similar extent. On the contrary, the reduction in the number of neurons immunoreactive for nerve growth factor receptor p75, which in control brains is highly co-localized with the expression of choline acetyltransferase, was much less pronounced and reached only 20-30%. The loss of choline acetyltransferase expression was associated with a cellular hypertrophy. Neurons which had survived the neurotoxic damage, furthermore, showed a remodelling of the dendritic organization which was quantitatively investigated after Golgi impregnation. This process of dendritic reorganization was mainly characterized by an increase in number and length of terminal dendritic segments. The results indicate that under the conditions of the present paradigm of chronic neurodegeneration, a certain number of cholinergic neurons persists in a form where they lost their ability to express detectable amounts of choline acetyltransferase messenger RNA and the enzyme protein. Persisting neurons, however, show both expression of nerve growth factor receptor p75 and signs of perikaryal and dendritic growth. It might, therefore, be hypothesized that chronic degeneration of cholinergic basal forebrain neurons triggers reactive attempts of repair which involve the action of trophic factors such as nerve growth factor.

The N-terminal portion of growth inhibitory factor is sufficient for biological activity

To determine its active site, growth inhibitory factor (GIF), a central nervous system-specific metallothionein-like protein, was digested with trypsin followed by Staphylococcus aureus protease V8 digestion. Of 5 peptide fragments separated from trypsin-digested GIF by reverse-phase high pressure liquid chromatography and gel filtration, only GIF1-26 or longer peptides showed growth inhibitory activity on cortical neurons in culture. A shorter peptide, GIF5-23, which was obtained by further digestion of GIF1-26 with V8 protease, also showed growth inhibitory activity. However, a synthetic peptide corresponding to GIF5-23 did not show growth inhibitory activity. Metal-free GIF1-26 prepared by acid treatment showed a similar level of growth inhibitory activity to that of metal-containing GIF1-26, indicating that metal in the peptide does not affect the activity. Treatment of metal-free GIF1-26 with beta-mercaptoethanol resulted in the loss of activity. The CD spectrum of beta-mercaptoethanol-treated metal-free GIF1-26 was different from that of nontreated metal-free GIF1-26. These results indicate that the N-terminal portion of GIF is required for growth inhibitory activity and that folding of the peptide via S-metal bonding is critical for biological activity.

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.

Nerve growth factor and Alzheimer's disease

Nerve growth factor (NGF) is a well-characterized protein that exerts pharmacological effects on a group of cholinergic neurons known to atrophy in Alzheimer's disease (AD). Considerable evidence from animal studies suggests that NGF may be useful in reversing, halting, or at least slowing the progression of AD-related cholinergic basal forebrain atrophy, perhaps even attenuating the cognitive deficit associated with the disorder. However, many questions remain concerning the role of NGF in AD. Levels of the low-affinity receptor for NGF appear to be at least stable in AD basal forebrain, and the recent finding of AD-related increases in cortical NGF brings into question whether endogenous NGF levels are related to the observed cholinergic atrophy and whether additional NGF will be useful in treating this disorder. Evidence regarding the localization of NGF within the central nervous system and its presumed role in maintaining basal forebrain cholinergic neurons is summarized, followed by a synopsis of the relevant aspects of AD neuropathology. The available data regarding levels of NGF and its receptor in the AD brain, as well as potential roles for NGF in the pathogenesis and treatment of AD, are also reviewed. NGF and its low affinity receptor are abundantly present within the AD brain, although this does not rule out an NGF-related mechanism in the degeneration of basal forebrain neurons, nor does it eliminate the possibility that exogenous NGF may be successfully used to treat AD. Further studies of the degree and distribution of NGF within the human brain in normal aging and in AD, and of the possible relationship between target NGF levels and the status of basal forebrain neurons in vivo, are necessary before engaging in clinical trials.

Neurotrophic factors in memory disorders

Neurotrophic factors are target-derived proteins capable of affecting survival, target innervation, and/or function of neuronal cell populations. These factors are structurally and functionally related to the classical neurotrophic molecule nerve growth factor (NGF) and resemble a genetic family called neurotrophins. Besides NGF and brain-derived neurotrophic factor (BDNF), there is little knowledge whether these neurotrophins play a pathophysiological role in dementing brain disease(s). BDNF-mRNA levels are reported to be decreased in the hippocampus of individuals with Alzheimer's disease (AD). Decreased NGF production does not seem to play a causal role both in age-related cognitive impairment and AD which is usually associated with neurodegenerative processes in the cholinergic basal forebrain system. However, there are several experimental indications that NGF might be of importance for the stimulation of compensatory changes and repair mechanisms; given in pharmacological dose, NGF might be of therapeutical benefit, as reported in a preliminary clinical case study. Thus, the availability of sufficient quantities of recombinant human neurotrophins should allow comprehensive research programs in future.

Trophic factors during normal brain aging and after functional damage

There is an increasing body of information concerning the physiological role of several target-derived neurotrophic proteins that are structurally and functionally related to the classical neurotrophic molecule NGF and which resemble a genetic family called neurotrophins. However apart from NGF, there is little knowledge about the pathophysiological role of these neurotrophins concerning aging or dementia. To our present knowledge, decreased NGF production does not seem to play a causal role in age-related cognitive impairment which is usually associated with neurodegenerative processes in the cholinergic basal forebrain system. However, there are several experimentally found indications that NGF might be of importance in the stimulation of compensatory changes and repair mechanisms. Moreover, recent findings suggest that disturbances in cerebral glucose metabolism may play an important role in cognitive disabilities during normal aging and also in dementia disorders such as Alzheimer's disease. Intracerebroventricular (ICV) injection of streptozotocin (STZ) has been reported to decrease cerebral glucose utilization and energy metabolism and to impair passive avoidance learning in adult rats. One week after ICV STZ treatment, NGF content was significantly decreased in the septal region, where NGF-responsive cell bodies are known to be located and where NGF exerts its neurotrophic action after retrograde transport from NGF-producing targets. In contrast, NGF levels were increased within 3 weeks after ICV STZ treatment by about the same magnitude as has been observed for aged learning-impaired rats in the target regions for the basal forebrain cholinergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)