Synaptic pathology in Alzheimer's disease: immunological data for markers of synaptic and large dense-core vesicles

Synapse loss may be a minor contributor to decreased regional cerebral blood flow in Alzheimer disease

In this study we tested the hypothesis that synapse loss contributes to decreased regional cerebral blood flow (rCBF) in Alzheimer disease (AD). We compared antemortem rCBF and postmortem analysis of synaptophysin, as a measure of synapse loss, in 13 cases of AD. rCBF studies were performed using inhaled xenon gas (Xe-133), which yields quantitative results. Synapse loss was evaluated in postmortem brain samples using an enzyme-linked immunosorbant assay (ELISA) that measures synaptophysin, with results expressed as picomoles synaptophysin/10 mg brain. Synaptophysin was expressed either as concentration (QS method) or as the ratio of the concentration to the combined results in frontal, temporal and parietal lobe (RS method). There was no correlation between synapse loss and rCBF using the QS method and only borderline significance between right SPECT and right temporal synaptophysin using the RS method. The results of this study suggest that synapse loss may be a minor contributor to the decreased rCBF observed in AD.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Intracellular deposition, microtubule destabilization, and transport failure: an "early" pathogenic cascade leading to synaptic decline

Protein deposition is a common event in age-related neurological diseases that are characterized by neuronal dysfunction and eventual cell death. Here, cultured hippocampal slices were infused with the lysosomal disrupter chloroquine to examine the link between abnormal protein processing/deposition and early synaptopathogenesis. Tau species of 55 to 69 kDa increased over several days of treatment with chloroquine, while the protein and message levels of synaptic markers were selectively reduced. Neurons of subfields CA1, CA3, and dentate gyrus accumulated protein deposits recognized by antibodies against paired helical filaments and ubiquitin, and this was accompanied by tubulin fragmentation and deacetylation. The deposition filled the basal pole of pyramidal neurons, encompassing the area of the axon hillock and initial dendritic branching but without causing overt neuronal atrophy. Neurons containing the polar aggregates exhibited severely impaired transport along basal dendrites. Transport capability was also lost along apical dendrites, the opposite direction of deposited material in the basal pole; thus, perpetuating the problem beyond physical blockage must be the associated loss of microtubule integrity. These data indicate that transport failure forms a link between tau deposition and synaptic decline, thus shedding light on how protein aggregation events disrupt synaptic and cognitive functions before the ensuing cellular destruction.

Synapse loss is greater in presenile than senile onset Alzheimer disease: implications for the cognitive reserve hypothesis

In the past, 'Alzheimer disease' (AD) referred to pathologic AD with clinical onset of dementia in the presenium, while 'senile dementia of the Alzheimer type' (SDAT) referred to senile onset AD. Because AD appears clinically homogeneous regardless of age of onset, the two subtypes in more recent years have not been distinguished. Pathologic differences have been noted, but synapse loss has not previously been compared between the two groups. Hypothesizing that synapse loss would be greater in presenile onset than senile onset AD, we compared synapse loss, as well as Alzheimer pathology in presenile and senile onset AD, using an ELISA method to quantify synaptophysin. Synaptophysin was significantly lower in presenile than senile AD in right frontal and bilateral parietal lobes. Neuritic plaque counts were significantly higher in presenile than senile AD in bilateral frontal and parietal lobes. Semi-quantitative evaluation of neurofibrillary tangles revealed significantly more tangles in bilateral frontal and parietal lobes in presenile than senile AD. Brain weight was significantly lower in presenile than senile AD. The differences in synapse loss and Alzheimer-type pathology in presenile and senile onset AD support the hypothesis that 'cognitive reserve' protects the human brain from neurodegenerative disease.

Maintained synaptophysin immunoreactivity in Tg2576 transgenic mice during aging: correlations with cognitive impairment

Regional loss of synapses, particularly within the neocortex and hippocampus, is characteristic of Alzheimer's Disease (AD) and strongly correlated with extent of cognitive impairment. The Tg2576 transgenic mouse model of AD develops Abeta-containing neuritic plaques by 10-16 months of age and shows cognitive impairment in several tasks. In the present study, synaptophysin immunoreactivity (SYN-IR; a marker for synaptic terminals) was evaluated in the neocortex and hippocampus of behaviorally-tested Tg2576 transgenic (Tg+) mice aged 3, 9, 14, and 19 months of age. In control non-transgenic (Tg-) mice, SYN-IR in both neocortex and hippocampus tended to decrease with age, while SYN-IR in Tg+ mice was maintained with age. Thus, 19M Tg+ mice exhibited significantly greater synaptophysin immunostaining compared to 19M Tg- mice in both inner and outer neocortical regions, as well as in the dentate gyrus' outer molecular layer and polymorphic layer. Over all four age groups collectively, outer cortical SYN-IR was also greater in Tg+ compared to Tg- mice. Multiple factors could be responsible for maintained SYN-IR in aged Tg+ mice, including compensatory changes in synaptic morphology and staining of dystrophic neuritics associated with Abeta deposition. For all animals combined (Tg+ and Tg-), as well as for aged 19M animals alone, hippocampal SYN-IR was correlated with impaired acquisition and spatial reference memory in the Morris water maze task, suggestive that elevated hippocampal SYN-IR is a manifestation of pathophysiologic synaptic processing within the hippocampus. Also for 19M animals alone, hippocampal SYN-IR was highly correlated with impaired visible platform recognition, indicative that elevated SYN-IR is linked to visual agnosia. The results of this study are consistent with the premise that maintained SYN-IR in Tg2576 mice during aging is associated with impaired synaptic function, resulting in cognitive deficits.

Synaptic proteins in Alzheimer's disease

Chromogranin A, chromogranin B, and secretogranin II are acidic proteins which are stored in large dense core vesicles of neurons. An antiserum, raised against a synthetic peptide (PE-11), present in the chromogranin B molecule, and an antiserum raised against secretoneurin contained in the secretogranin II sequence, was used to localize these peptides together with chromogranin A in the human hippocampal formation. The distribution of these peptides was investigated in Alzheimer's disease and compared to control subjects. Chromogranin A, chromogranin B, and secretogranin II are distinctly distributed with an overlap in their distribution patterns. They were only detected in neuronal structures. The highest density of immunoreactivity was found for chromogranin B. A layer specific distribution was especially obvious in the inner molecular layer of the dentate gyrus as secretoneurin-like immunoreactivity was restricted to its innermost part whereas that of chromogranin B was highly concentrated throughout the inner molecular layer. In Alzheimer's disease, about 10 to 20% of the amyloid-immunoreactive plaques contained either chromogranin A, chromogranin B or secretoneurin. The density of secretoneurin-and chromogranin B-like immunoreactivity was significantly reduced in the inner molecular layer of the dentate gyyrs, the CA1 area, the subiculum and in layers I, III and V of the entorhinal cortex. The present study demonstrates that chromogranin peptides are markers for human hippocampal pathways. Thee are particularly suitable to study nerve fibers, terminating at the inner molecular layer of the dentate gyrus. Chromogranin peptides have a potential as neuronal markers for synaptic degeneration in Alzheimer's disease.

Synaptic pathology in prefrontal cortex is present only with severe dementia in Alzheimer disease

Synaptic pathology is proposed to be integral to the clinical expression of Alzheimer disease (AD). Most studies have assessed only the vesicle protein synaptophysin as a measure of synaptic integrity. The interrelationships of synaptophysin, other presynaptic proteins, the cholinergic system, and severity of dementia in AD remain unclear. We studied the presynaptic proteins synaptophysin, syntaxin and SNAP-25, along with choline acetyltransferase (ChAT) activity in prefrontal cortex (BA 46) samples from 18 subjects with AD and 16 controls. Mean values of presynaptic protein immunoreactivities were significantly reduced, by 21%-28%, and ChAT activity was reduced by 41% in the AD groups. Synaptic protein immunoreactivity and ChAT activity were correlated with Mini-Mental State Examination scores obtained 1 yr prior to death. When AD cases were subgrouped into mild/moderate and severe illness at time of death, all differences in presynaptic proteins and ChAT activity were significant between controls and severe cases. However, no significant differences were detected in BA 46 between controls and mild/moderate cases. Considerable synaptic reserve or plasticity remains in BA 46 until the late stages of AD. Synaptophysin and ChAT appear to be more vulnerable in severe AD than are syntaxin or SNAP-25.

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".

Cortical synapse loss in progressive supranuclear palsy

Cortical synapse loss, the probable substrate of cognitive impairment in Alzheimer disease (AD), has not previously been evaluated in progressive supranuclear palsy (PSP). Hypothesizing that synapse loss would be greater in demented than non-demented PSP patients, we examined synaptophysin concentrations in 8 cases of PSP (5 demented and 3 nondemented cases). We found a decrease in mean synaptophysin concentration in these 8 cases in frontal, temporal, and parietal lobes, and in cerebellum, compared to the means in corresponding lobes of 16 controls. The decreases were similar to those in 28 cases of AD, but not as great. We determined synaptophysin concentration from motor cortex in only 4 of our PSP cases, 2 demented and 2 non-demented. The average concentrations in these 4 cases were lower than in AD motor cortex; both were lower than controls. When demented and non-demented PSP cases were compared, neocortical synaptophysin concentrations in non-demented PSP cases were lower than in demented cases. There appears to be a link between AD and PSP, in that synapse loss is found in both. However, the basis and significance of the prominent neocortical synapse loss in PSP, especially in non-demented subjects, remain to be explored.

Synaptic loss is accompanied by an increase in synaptic area in the dentate gyrus of aged human apolipoprotein E4 transgenic mice

To investigate the relationship between the three isoforms of apolipoprotein E (E2, E3 and E4) and the integrity of the synaptic circuitry in the dentate gyrus of the hippocampus, we have estimated the synapse per neuron ratio and mean apposition zone area per synapse at the electron microscope level in the dentate gyrus of apolipoprotein E knockout and human apolipoprotein E transgenic mice aged six to 24months. During ageing, only in human apolipoprotein E4 mice was there a decrease in synapse per neuron ratio, accompanied by an increase in synaptic size. When these mice were compared with human apolipoprotein E2, apolipoprotein E knockout and wild-type mice at old age, they displayed the lowest synapse per neuron ratio, but similar apposition zone area. In contrast, as in our previous study, aged apolipoprotein E knockout mice did not show any sign of synaptic degeneration. The functional consequences of such morphological changes remain to be determined. However, if such age-related loss of synapses occurred in the brain of Alzheimer apolipoprotein E4 patients, they might be additive to pathological processes and could contribute to greater cognitive impairment.

Properties of NACP/alpha-synuclein and its role in Alzheimer's disease

The precursor of the non-amyloid beta/A4 protein (non-Abeta) component of Alzheimer's disease amyloid (NACP)/alpha-synuclein is the human homologue of alpha-synuclein, a member of a protein family which includes alpha-, beta- and gamma-synuclein. This protein is thought to be involved in neuronal plasticity because of its unique expression, mainly in the telencephalon during maturation. Consequently, disarrangement of NACP/alpha-synuclein might disrupt synaptic activity, resulting in memory disturbance. Previous studies have shown that damage to synaptic terminals is closely associated with global cognitive impairment and is an early event in the pathogenesis of Alzheimer's disease. Although the relationship between synaptic damage and amyloidogenesis is not clear, some proteins at the synaptic site have been implicated in both neuronal alteration and amyloid formation. Indeed, abnormal accumulation of both NACP/alpha-synuclein and Abeta precursor protein occurs at synapses of Alzheimer's patients. Other evidence suggests that NACP/alpha-synuclein is a component of the Lewy bodies found in patients with Parkinson's disease or dementia with Lewy bodies, and that a point mutation in this protein may be the cause of familial Parkinson's disease. Consequently, abnormal transport, metabolism or function of NACP/alpha-synuclein appears to impair synaptic function, which induces, at least in part, neuronal degeneration in several neurodegenerative diseases.

Selective regional loss of exocytotic presynaptic vesicle proteins in Alzheimer's disease brains

We tested whether regional or selective alterations in presynaptic proteins occur in Alzheimer's disease (AD) and correlate with tests of cognitive function. We measured the levels of seven presynaptic proteins (synaptobrevin, synaptotagmin, SNAP-25, syntaxin, SV2, Rab3a, and synapsin I) by immunoblotting in postmortem tissue from four brain regions (hippocampus, entorhinal cortex, caudate nucleus, and occipital cortex). Three subject groups were studied: AD, possible/early AD (p-AD), and age-matched controls. Synaptobrevin and synaptotagmin were significantly reduced (29%, P<0.08; 38%, P<0. 07) in hippocampus in p-AD compared to controls. In definite AD compared to controls, selective regional reductions in vesicle proteins were found: synaptobrevin (46%, P<0.05), synaptotagmin (52%, P<0.01), and Rab3a (30%, P<0.05) in hippocampus; synaptobrevin (31%, P<0.01), synaptotagmin (15%, P<0.05), and Rab3a (44%, P<0.05) in entorhinal cortex. In contrast, the levels of two vesicle proteins (synapsin I and SV2) and two presynaptic membrane proteins (syntaxin and SNAP-25) were similar to controls. Synaptobrevin was the only vesicle protein reduced in AD in all four brain regions (occipital cortex 37%, P<0.05; caudate nucleus 31%, P<0.05). By univariate analysis of all cases, Mini-Mental State Examination, Blessed (BIMC) and Free Recall scores were strongly correlated with reduced levels of synaptic vesicle proteins synaptobrevin, synaptotagmin, and Rab3a in hippocampus and entorhinal cortex. These results suggest that there are selective and early defects in presynaptic vesicle proteins, but not synaptic plasma membrane proteins in AD and that defects correlate with cognitive dysfunction in this disease.

Agrin and microvascular damage in Alzheimer's disease

Heparan sulfate proteoglycans (HSPGs) are ubiquitously present within the perivascular basement membrane, and have been shown to be altered in patients with Alzheimer's Disease (AD). Although the HSPG agrin clearly orchestrates the differentiation of the neuromuscular junction, its role in the brain remains unclear. Growing evidence suggests that agrin may be an important vascular basement membrane (VBM)-associated HSPG. In previous studies, we demonstrated that agrin is present throughout the brain microvasculature, as well as in neuronal cell bodies. AD brains exhibited fragmentation of VBM-associated agrin. Agrin immunoreactivity was also seen within senile plaques and neurofibrillary tangles. These changes were accompanied by the appearance of an additional pool of insoluble agrin. In the present study, we provide further evidence for microvascular damage in AD, by examining the distribution of agrin and laminin within the VBM, and by measuring the agrin concentration within hippocampus and prefrontal cortex. Furthermore, we assessed blood-brain-barrier (BBB) leakage by examining the perivascular distribution of prothrombin immunoreactivity. Soluble agrin levels were increased approximately 30% in Braak stage III-VI AD patients relative to age-matched controls. Furthermore, agrin and laminin exhibited identical patterns of VBM fragmentation in AD and colocalized with beta-amyloid in senile plaques. Microvascular changes were associated with the appearance of perivascular prothrombin immunoreactivity. Our data suggest that agrin is an important VBM-associated HSPG in the brain and that agrin levels are altered in association with microvascular damage in AD.

Agrin in Alzheimer's disease: altered solubility and abnormal distribution within microvasculature and brain parenchyma

Agrin is a heparan sulfate proteoglycan that is widely expressed in neurons and microvascular basal lamina in the rodent and avian central nervous system. Agrin induces the differentiation of nerve-muscle synapses, but its function in either normal or diseased brains is not known. Alzheimer's disease (AD) is characterized by loss of synapses, changes in microvascular architecture, and formation of neurofibrillary tangles and senile plaques. Here we have asked whether AD causes changes in the distribution and biochemical properties of agrin. Immunostaining of normal, aged human central nervous system revealed that agrin is expressed in neurons in multiple brain areas. Robust agrin immunoreactivity was observed uniformly in the microvascular basal lamina. In AD brains, agrin is highly concentrated in both diffuse and neuritic plaques as well as neurofibrillary tangles; neuronal expression of agrin also was observed. Furthermore, patients with AD had microvascular alterations characterized by thinning and fragmentation of the basal lamina. Detergent extraction and Western blotting showed that virtually all the agrin in normal brain is soluble in 1% SDS. In contrast, a large fraction of the agrin in AD brains is insoluble under these conditions, suggesting that it is tightly associated with beta-amyloid. Together, these data indicate that the agrin abnormalities observed in AD are closely linked to beta-amyloid deposition. These observations suggest that altered agrin expression in the microvasculature and the brain parenchyma contribute to the pathogenesis of AD.

Expression of proteins linked to exocytosis and neurotransmission in patients with Creutzfeldt-Jakob disease

In order to characterize synaptic involvement in human spongiform encephalopathies, the expression of synaptic vesicle-associated proteins, synaptophysin and synapsin-I, and presynaptic plasma membrane proteins, synaptosomal-associated protein of 25 kDa (SNAP-25) and syntaxin-I, was examined in the brains of four patients who had suffered from sporadic Creutzfeldt-Jakob disease. Nerve cell loss, spongiform degeneration, astrocytosis, and deposition of prion protein (PrP) were observed in the cerebral cortex in every case. Decreased immunoreactivity for synaptophysin, synapsin-I, SNAP-25, and syntaxin-I was observed in the cerebral cortex in every case, thus showing generalized reduction of proteins involved in exocytosis of synaptic vesicles in the brains of patients with spongiform encephalopathy. Upregulation of synaptophysin and SNAP-25, a feature associated with beta A4 deposits in Alzheimer's disease (AD), was not observed in associated with PrP deposition. The present results indicate that synaptic pathology is a major event in spongiform encephalopathy, and suggest that synaptic loss, together with neuron loss and selective involvement of certain populations of local-circuit neurons, as shown in other studies, may account for the dramatic neurological decay and for the main neurological symptoms in patients with CJD.

Neurotrophin activation of catecholamine storage vesicle protein gene expression: signaling to chromogranin a biosynthesis

Nerve growth factor differentiates precursor cells into sympathetic neurons. Does acquisition of a "neuronal" phenotype after nerve growth factor involve biosynthesis of chromogranin A, the major soluble protein in chromaffin granule cores? Nerve growth factor activated chromogranin A gene expression 7.6-fold in PC12 pheochromocytoma cells, and similarly activated PC12-transfected mouse, rat or human chromogranin A promoter/reporter constructs. Chromogranin A promoter 5'-deletions narrowed the nerve growth factor response element to a region from - 77 to - 61 bp upstream of the cap site, a region containing the chromogranin A cyclic AMP response element (TGACGTAA). Three different site-directed mutations of the cyclic AMP response element each reduced the nerve growth factor effect by >90%. Transfer of the cyclic AMP response element to a heterologous (thymidine kinase) promoter activated that promoter approximately 5-fold after nerve growth factor, while transfer of a cyclic AMP response element point-gap mutant (TGA-GTAA) to a heterologous promoter abolished the nerve growth factor effect. These findings indicate that the cyclic AMP response element in cis is, at least in part, both necessary and sufficient to activate the chromogranin A gene. Chemical blockade of the nerve growth factor receptor TrkA or the mitogen-activated protein kinase pathway component MEK substantially diminished nerve growth factor-induced expression of chromogranin A. By contrast, the response of chromogranin A to nerve growth factor was not impaired after blockade of phospholipase C-gamma or phosphoinositide-3 kinase. Chemical blockade of TrkA, Ras, MEK or mitogen-activated protein kinase similarly inhibited nerve growth factor activation of chromogranin A. Expression of constitutively activated Ras, Raf or MEK mutants increased chromogranin A promoter activity. Expression of dominant negative (inhibitory) mutants of Sos, Ha-Ras, Rafl, mitogen-activated protein kinase, ribosomal protein S6 serine kinase II (CREB kinase) or CREB (KCREB) each inhibited the nerve growth factor-induced increase in chromogranin A promoter activity. Thus, each component of the mitogen-activated protein kinase pathway is crucially involved in relaying the nerve growth factor signal in trans to the chromogranin A gene, in the following proposed sequence: nerve growth factor --> TrkA --> Shc/Grb2/Sos --> Ras --> Raf --> MEK --> mitogen-activated protein kinase --> ribosomal protein S6 serine kinase II --> CREB cyclic AMP response element.

Neuropathology of Alzheimer's disease: a critical update

The unequivocal diagnosis of Alzheimer's disease (AD) rests on histopathological evidence at brain autopsy or biopsy. The morphology of AD includes cerebral atrophy, deposition of beta A4 amyloid (A beta) (senile plaques and amyloid angiopathy), neuritic changes (neuritic plaques, neurofibrillary tangles (NFT) and neuropil threads) with formation of paired helical filaments (PHF) containing polymerized hyperphosphorylated tau protein triplet, causing disruption of the neuronal cytoskeleton with loss of synapses and neurons, with altered cortico-cortical connectivity, leading to disconnection of the cerebral cortex. Defining criteria for the morphologic diagnosis of AD is difficult due to the phenotypic heterogeneity of the disease, the absence of specific markers, and overlap of AD morphology with that observed in non-demented elderly individuals. This gray zone between normal to pathologic aging and full-fledged AD represents an important diagnostic problem and should be overcome by better standardized criteria that will allow to minimize interrater and interlaboratory variability in the diagnosis of AD. Current criteria for the morphologic diagnosis of AD are based on (semi)quantitative assessment of diffuse and neuritic plaques (NIA), exclusively neuritic plaques (CERAD), plaques and NFT in neocortex and hippocampus (Tierney et al., 1988), and staging of hierarchic spreading of neuritic AD changes (Braak and Braak, 1991); all of them have weaknesses and need to be revalidated. Multivariant analysis of an autopsy series of elderly subjects revealed significant correlations between psychostatus and both the CERAD criteria and Braak staging. Recent recommendations of the NIA-Reagan Institute for the morphologic diagnosis of AD are presented. Although the role of plaques and NFT in the pathogenesis of AD remains undetermined, clinicopathological correlative studies have shown that both lesions, if present in sufficient numbers, particularly in the neocortex, are considered the best morphological signposts for AD. Recent studies on neuron death in AD that, at least in part, appears different from classical apoptosis and may precede the symptomatic stage of AD, have shown varying results indicating only indirect relationship between DNA fragmentation and both A beta deposition and NFTs. Both these AD-typical markers appear to increase the risk of cells to degenerate, but are not the sole responsibles of the degenerative process in AD, the basic mechanisms of which remain to be elucidated.

Hippocampal neuron and synaptophysin-positive bouton number in aging C57BL/6 mice

A loss of hippocampal neurons and synapses had been considered a hallmark of normal aging and, furthermore, to be a substrate of age-related learning and memory deficits. Recent stereological studies in humans have shown that only a relatively minor neuron loss occurs with aging and that this loss is restricted to specific brain regions, including hippocampal subregions. Here, we investigate these age-related changes in C57BL/6J mice, one of the most commonly used laboratory mouse strains. Twenty-five mice (groups at 2, 14, and 28-31 months of age) were assessed for Morris water-maze performance, and modern stereological techniques were used to estimate total neuron and synaptophysin-positive bouton number in hippocampal subregions at the light microscopic level. Results revealed that performance in the water maze was largely maintained with aging. No age-related decline was observed in number of dentate gyrus granule cells or CA1 pyramidal cells. In addition, no age-related change in number of synaptophysin-positive boutons was observed in the molecular layer of the dentate gyrus or CA1 region of hippocampus. We observed a significant correlation between dentate gyrus synaptophysin-positive bouton number and water-maze performance. These results demonstrate that C57BL/6J mice do not exhibit major age-related deficits in spatial learning or hippocampal structure, providing a baseline for further study of mouse brain aging.

Mechanisms of synaptic pathology in Alzheimer's disease

Neurodegenerative disorders are characterized by damage to selective neuronal populations that could be followed or preceded by synaptic injury. Therefore, specific mutations in and other alterations of synaptic proteins might lead to particular neurodegenerative diseases. The predominant hypothesis is that these mutations result in an increased production of amyloid beta-protein 1-42 which acts as a neurotoxin. However, it could also be postulated that amyloid precursor protein might play an important role in synaptic function and neuronal maintenance and that its abnormal activity may lead to neurodegeneration. Recent studies have shown that amyloid precursor protein has an important role in regulating glutamate levels at the synaptic site by modulating the activity of glutamate transporters. The objectives of this manuscript are to highlight recent data supporting the hypothesis that neurodegeneration in Alzheimer's disease might be the combined result of abnormal protective activity of amyloid precursor protein and amyloid beta-protein toxicity.

The neuropathological diagnosis of Alzheimer disease

The unequivocal diagnosis of Alzheimer disease (AD) rests on histopathological evidence at brain autopsy or biopsy. Although the histological features of AD are well known, defining criteria for the morphological diagnosis of AD is difficult due to the phenotypical heterogeneity of the disease, absence of specific markers, and overlap of AD pathology with that observed in non-demented elderly individuals. This gray zone between normal to pathological aging and full-fledged AD represents an important diagnostic problem and should be overcome by better standardized criteria that will allow to minimize interrater and interlaboratory variability in the diagnosis of AD. Current criteria for the neuropathological diagnosis of AD are based on age-related (semi)quantitative assessment of "senile" plaques (NIA criteria), neuritic plaques (CERAD), plaques and neurofibrillary tangles in neocortex and hippocampus (Tierney et al., 1988), and staging of hierarchic spreading of neuritic AD changes in particular, neurofibrillary tangles (Braak and Braak, 1991). All these algorithms have some weaknesses and do not recognize the various subtypes of AD. Multivariant analysis of an autopsy series of elderly subjects revealed significant correlations between psychostatus assessed by the Mini-Mental State and both the CERAD criteria and Braak staging. Although the role of plaques and tangles in the pathogenesis of AD and their relationship to both neuronal loss and dementia remain to be elucidated, clinicopathological studies have shown that both lesions, if present in sufficient numbers, particularly in the neocortex, are considered the best correlates for AD related dementia. Recent consensus recommendations of the NIA- and Reagan Institute Working Group for the morphological diagnosis of AD consider AD as a heterogenous clinicopathological entity. After exclusion of other causes of dementia, the likelihood that AD accounts for dementia is considered high, intermediate or low according to the frequency of neuritic AD lesions with regard to both the CERAD criteria and Braak staging. The evaluation of small autopsy series according to these criteria demonstrated their easy and rapid application in AD and non-demented subjects, with much less reliability for other dementing disorders.

Temporal cortex synaptophysin mRNA is reduced in Alzheimer's disease and is negatively correlated with the severity of dementia

We measured synaptophysin mRNA in neocortical tissue from 7 prospectively assessed, pathologically verified normal individuals, 17 subjects with Alzheimer's disease (AD), and 13 subjects with a non-AD dementia. In temporal cortex (Brodmann area 21), synaptophysin mRNA was decreased in AD and non-AD dementia groups compared to controls. The loss was also present relative to polyadenylated mRNA content. Synaptophysin mRNA signal correlated negatively with the degree of dementia and negatively with the pathological severity of AD. In occipital cortex (Brodmann area 17) there were no differences between groups nor clinicopathological correlations. These data extend the evidence for a regional synaptic pathology in AD which affects synaptic protein gene expression by temporal cortex neurons.

Neurochemical dissection of synaptic pathology in Alzheimer's disease

Synaptic pathology has attained increasing attention as being central in the pathogenesis of Alzheimer's disease (AD). To address the question whether synaptic pathology in AD involves the whole synapse, or is limited to specific components thereof, we studied three different synaptic vesicle proteins (rab3a, synaptotagmin, synaptophysin) and also the presynaptic membrane protein GAP-43 and the postsynaptic protein neurogranin. The material included postmortem brain tissue (frontal cortex, hippocampus, and cerebellum) from 8 patients with early-onset AD (EAD), 11 patients with late-onset AD (LAD), 6 patients with vascular dementia (VAD), and 9 control subjects. A reduction of all synaptic proteins was found in AD, more pronounced in EAD than in LAD, in both the frontal cortex (EAD 30% to 70% vs. LAD 82% to 88% of control value) and hippocampus (EAD 22% to 82% vs. LAD 76% to 89% of control value), whereas only minor changes were found in VAD. The finding that all synaptic proteins were reduced in AD suggests a degeneration and loss of whole synaptic elements that are more pronounced in EAD than in LAD.

Synaptic loss reflected by secretoneurin-like immunoreactivity in the human hippocampus in Alzheimer's disease

Secretoneurin is a recently described peptide derived by endoproteolytic processing from secretogranin II, previously named chromogranin C. In this study, we have investigated the distribution of secretoneurin-like immunoreactivity in the human hippocampus in controls and in Alzheimer's disease patients, and compared the staining pattern to that of calretinin. Secretoneurin-like immunoreactivity is present throughout the hippocampal formation. At the border of the dentate molecular layer and the granule cell layer, a band of dense secretoneurin immunostaining appeared. In this part, as in the area of the CA2 sector, the high density of secretoneurin-immunoreactivity coincided with calretinin-like immunoreactivity. The mossy fibre system displayed a moderate density of secretoneurin-immunoreactivity. In the entorhinal cortex, a particularly high density of secretoneurin-immunoreactivity was observed. The density of secretoneurin-like immunoreactivity was significantly reduced in the innermost part of the molecular layer and in the outer molecular layer of the dentate gyrus in Alzheimer's disease. For calretinin-like immunoreactivity, a less pronounced decrease was found in the innermost part of the molecular layer. About 40-60% of neuritic plaques were secretoneurin-immunopositive. This study shows that secretoneurin is distinctly distributed in the human hippocampus and that significant changes of secretoneurin-like immunoreactivity occur in Alzheimer's disease, reflecting synaptic loss.

Morphological substrates of dementia in parkinsonism. A critical update

Dementia in parkinsonism is caused by a variety of central nervous system (CNS) lesions, of which the molecular and pathogenic causes are poorly understood but probably include: 1. Degeneration of subcortical ascending systems with neuronal losses in dopaminergic, noradrenergic, serotonergic, cholinergic or multiple systems including the amygdyloid nucleus; 2. limbic and/or cortical Alzheimer and/or Lewy body pathologies, with loss of synapses and neurons, and 3. a combination of these lesions or additional CNS pathologies. In general, degeneration of subcortical neuronal networks appears insufficient to induce severe mental decline although, occasionally, cognitive impairment occurs without apparent cortical lesions. On the other hand, neuritic cortical Alzheimer change showing similar or differential distribution compared to Alzheimer's disease (AD) displays a significant linear correlation with dementia in Parkinsonism. Plaques can be associated with cortical Lewy bodies and, the contribution of each to dementing processes remains unresolved. In a consecutive autopsy series of 610 patients with parkinsonism, the total prevalence of retrospectively assessed dementia was 34.6%. In Parkinson's disease (PD) of the Lewy body type, it was 30.2%, mostly associated with other brain lesions, mainly AD, while only 3.5% of "pure" PD without additional brain pathologies were demented. There was no significant difference in age and duration of illness between demented and non-demented PD patients. Secondary parkinsonian syndromes showed a higher incidence of dementia (56.3%), again with predominant Alzheimer pathology which was present in 73% of the total of demented parkinsonian patients and in almost 82% of the demented PD cases in this series. The specific contribution of cortical and subcortical lesions to mental impairment in parkinsonism, their relationship to AD, and an etiology await further elucidation.

Proposals for re-evaluation of current autopsy criteria for the diagnosis of Alzheimer's disease

Defining criteria for the postmortem diagnosis of Alzheimer's disease (AD) has proven difficult due to the phenotypical heterogeneity of the disease, the absence of a specific disease marker and an overlap of AD neuropathology with that observed in a number of nondemented aged individuals. Even though the role of plaques and tangles in the pathogenesis of AD remains undetermined, a host of clinicopathological correlative studies have shown that both lesions, if present in sufficient numbers-particularly in the neocortex-are still to be considered the best morphological signposts for the disease. All currently used criteria for the neuropathologic diagnosis of AD have some weaknesses and need to be reestablished and revalidated. Multivariant analysis in a personal autopsy series of elderly subjects revealed significant correlations between psychostatus and both the CERAD criteria and Braak staging of neuritic Alzheimer-type lesions, and less concordance with the National Institutes of Aging and Tierney criteria. We propose a set of histopathologic diagnostic criteria for both definite and preclinical AD that rely on various constellations of both different types of plaques, except diffuse amyloid deposits, and neurofibrillary tangles, in allocortical and isocortical areas considering their topographic pattern. This set of criteria encompasses phenotypic variations of the pathology and takes into account the chronic, progressive course of AD. It allows the detection of preclinical disease in subjects in whom dementia is not reported and includes those cases in the morphological gray zone between "normal" aging and full-fledged AD that practicing neuropathologists consider the most problematic. The set of criteria includes guidelines concerning tissue sampling and processing, and standardized staining methods that should allow neurologists to minimize interrater and interlaboratory variability in the assessment of morphologic lesions and the diagnosis of AD.

The neurochemistry of Alzheimer's disease

The last 15-20 years have seen a wealth of studies to characterize the neurochemical abnormalities of Alzheimer's disease, in particular those involving the beta-amyloid and tau proteins, as well as more recently, apolipoprotein E4. This article provides a summary of the evidence for the involvement of these proteins in Alzheimer's disease pathogenesis based on postmortem brain and CSF studies.

Deficient glutamate transport is associated with neurodegeneration in Alzheimer's disease

The mechanisms of synapse damage in Alzheimer's disease (AD) are not fully understood. Deficient functioning of glutamate transporters might be involved in synaptic pathology and neurodegeneration by failing to clear excess glutamate at the synaptic cleft. In AD, glutamate transporter activity as assessed by D-[3H]aspartate binding is decreased; however, it is not clear to what extent it is associated with the neurodegenerative process and cognitive alterations. For this purpose, levels of D- and L-[3H]aspartate binding in midfrontal cortex were correlated with synaptophysin levels, brain spectrin degradation product levels, and clinical and neuropathological indicators of AD. Compared to control brains, AD brains displayed a 34% decrease in levels of D-[3H]aspartate binding, a 30% decrease in L-[3H]aspartate binding, and a 48% loss of synaptophysin immunoreactivity. Increased levels of brain spectrin degradation products correlated with a decrease in levels of D-[3H] and L-[3H]aspartate binding, and decreased levels of synaptophysin immunoreactivity. Levels of L-[3H]aspartate binding correlated with levels of synaptophysin immunoreactivity. These results suggest that decreased glutamate transporter activity in AD is associated with increased excitotoxicity and neurodegeneration, supporting the possibility that abnormal functioning of this system might be involved in the pathogenesis of synaptic damage in AD.

CSF of neuroleptic-naive first-episode schizophrenic patients: levels of biogenic amines, substance P, and peptides derived from chromogranin A (GE-25) and secretogranin II (secretoneurin)

Lumbar cerebrospinal fluid (CSF) was collected from controls and neuroleptic-naive patients with their first acute schizophrenic episode. The CSF was analyzed for several biogenic amines and their metabolites [dopamine,dihydroxyphenylacetic acid (DOPAC), noradrenaline, 5-hydroxytryptamine (5-HT), 5-hydroxyindolacetic acid (5-HIAA)]. For these transmitters, which are stored and secreted from synaptic vesicles, there was no significant difference between controls and schizophrenic patients. As constituents of large dense-core vesicles substance P (SP) and GE-25 (derived from chromogranin A)-and secretoneurin (derived from secretogranin 11)-immunoreactivities were determined. SP-like immunoreactivity levels did not differ between controls and patients; however, GE-25 was elevated and especially the GE-25/secretoneurin ratio was significantly (p < .001) higher in patients. Characterization of the immunoreactivities by high-performance liquid chromatography did not reveal any difference between patients (n = 3) and controls in the processing of the two proproteins chromogranin A and secretogranin II. These data indicate that proteolytic processing of the two widespread constituents of large dense-core vesicles, i.e., chromogranin A and secretogranin II, is not altered in schizophrenic patients. The increase in the chromogranin A /secretoneurin ratio in schizophrenic patients deserves further investigation in order to elucidate its possible pathogenetic significance.

Patterns of synaptic and nerve cell pathology in Alzheimer's disease

Recent neuropathological evidence suggests that synapse pathology is the major correlate of cognitive decline in Alzheimer's disease (AD) patients, but also in other dementia syndromes. We suggest that synapse loss in AD-patients mainly reflects neuronal destruction in other iso- and allocortical areas as well as in brain stem nuclei. In addition an impaired compensatory synaptogenesis may contribute to the reduction in synaptic connectivity. The patterns of cell death in AD-brains determined by analysis of DNA-fragmentation in situ revealed significantly higher numbers of dying cells (neurons as well as glia cells) in AD-brains compared to controls. Amyloid deposition as well as neurofibrillary pathology apparently do not induce cell death directly, but may increase the risk of cells to die in response to additional minor metabolic insults. We propose that multiple pathogenetic factors are involved in the reduction of synaptic connectivity in AD-brains, which finally is reflected in the decline of cognitive functions.

The synaptic protein NACP is abnormally expressed during the progression of Alzheimer's disease

NACP, the precursor of non-A beta component of Alzheimer's disease (AD) amyloid (NAC), is a synaptic protein that could potentially be involved in AD. We studied, by dot-blot, NACP levels in the frontal cortex of AD cases staged according to severity of disease and correlated them with cognitive performance and neuropathological markers. Early AD cases showed one fold higher levels of NACP immunoreactivity (IR) compared to moderate and severe AD. Levels of NACP-IR were correlated with tangle counts (r = -0.305, P = 0.04) and Blessed score (r = -0.356, P = 0.01), but not with plaque counts (r = 0.132, P = 0.39). This study suggests that the abnormal accumulation of NACP during the early stages of AD might play an important role in the mechanisms of neurodegeneration and synaptic damage in AD.

Enhanced synaptophysin immunoreactivity in rat hippocampal culture by 5-HT 1A agonist, S100b, and corticosteroid receptor agonists

Serotonin (5-HT) has been shown to modulate brain maturation during development and adult plasticity. This effect in the whole animal may be due to activation of 5-HT1A receptors and a corresponding increases in S100b and corticosterone. Synaptophysin, an integral protein of the synaptic vesicle membrane that correlates with synaptic density and neurotransmitter release, is reduced by depletion of 5-HT in the cortex and hippocampus of the adult rat. Injections of a 5-HT1A agonist or dexamethasone can reverse the loss of synaptophysin immunoreactivity (IR). In this study we used morphometric analysis of synaptophysin-IR to study the effects of the 5-HT1A agonist, ipsapirone, and the neuronal extension factor, S100b on hippocampal neurons grown in a serum and steroid free media. Both compounds increased the synaptophysin-IR at doses previously established to be highly specific. Ipsapirone (10(-9)M) was more effective on neuronal cell bodies staining and S100b (10 ng/ml) was more effective in increasing the number of synaptophysin-IR varicosities on neuronal processes. In addition both types of corticosteroid receptor agonists, at previously established specific doses, Ru28362 (10(-8) M) and aldosterone (10(-9) M) produced smaller increases compared to control groups in both the cell body staining and the number of varicosities. The effect of these differentiating factors on the expression of synaptophysin-IR suggests multiple regulation sites for producing and maintaining pre-synaptic elements in the brain.

Synaptic pathology in Alzheimer's disease: relation to severity of dementia, but not to senile plaques, neurofibrillary tangles, or the ApoE4 allele

Alzheimer's disease (AD) is characterised by an increased number of senile plaques (SP) and neurofibrillary tangles (NFT) as compared with that found in non-demented individuals of the same age, and a marked degeneration and loss of synapses. One of the main risk-factors for the disorder is inheritance of the apolipoprotein E4 (ApoE4) allele. To further study the relation between these pathogenetic substrates for AD, we quantified the synaptic vesicle membrane protein rab3a in brain tissue from 19 patients with AD and 9 age-matched control subjects. Rab3a levels were reduced in AD, both in the hippocampus (60% of control level, p < 0.0001), and in the frontal cortex (68% of control level, p < 0.01), but not in the cerebellum (92% of control level). Within the AD group, lower rab3a levels were found both with increasing duration and severity of dementia. These findings further support that synaptic pathology is closely correlated to the clinical dementia in AD. In contrast, no significant correlations were found between SP counts and duration or severity of dementia, while higher NFT counts in the frontal cortex were found with increasing severity of dementia (r = 0.54, p < 0.05). There were no significant correlations between the rab3a level and SP or NFT counts, and by immunohistochemistry, reduced rab3a immunostaining was found throughout the neuropil in AD brain, without relation to SP or NFT. These findings suggest that the synaptic pathology in AD is not closely related to the presence of SP and NFT. No significant differences in rab3a levels were found in any brain region between AD patients possessing different numbers of the ApoE4 allele, suggesting that, although ApoE4 is A risk factor for earlier development of AD, the degree of synaptic pathology does not differ between patients with or without the ApoE4 allele.

Chromogranin A triggers a phenotypic transformation and the generation of nitric oxide in brain microglial cells

Chromogranin A is an ubiquitous 48,000 mol. wt secretory protein stored and released from many neuroendocrine cells and neurons. In human brain, chromogranin A is a common feature of regions that are known to be affected by various neurodegenerative pathologies such as Alzheimer's, Parkinson's and Pick's diseases. Brain degenerative areas are generally infiltrated by activated microglial cells, the resident macrophage cell population within the central nervous system. Here, we report that both recombinant human chromogranin A and chromogranin A purified from bovine chromaffin granules trigger drastic morphological changes in rat microglial cells maintained in culture. Microglial cells exposed to chromogranin A adopted a flattened amoeboid shape and, this change was associated with an accumulation of actin in the subplasmalemmal region, as observed by immunocytochemistry and confocal laser microscopy. In single microglial cells loaded with indo-1, chromogranin A elicited a rapid and transient increase in [Ca2+]i which preceded the reorganization of actin cytoskeleton. The activity of nitric oxide synthase was estimated by measuring the accumulation of nitrite in the culture medium. Both recombinant human chromogranin A and bovine chromogranin A triggered an important accumulation of nitrite comparable to that induced by lipopolysaccharide, a well-known activator of microglia. The effect of chromogranin A was dose dependent, inhibited by N omega-nitro-L-arginine methyl ester, a competitive inhibitor of nitric oxide synthase, and by cycloheximide, an inhibitor of protein synthesis. These findings suggest that chromogranin A induces an activated phenotype of microglia, and thus may have a role in the pathogenesis of neuronal degeneration in the brain.

The new neuropathology of degenerative frontotemporal dementias

The clinical features and recent developments in the neuropathology of frontotemporal dementia are reviewed. The five main neurodegenerative disorders that underlie the clinical syndrome of frontotemporal dementia are distinguished using immunohistochemistry with antisera to ubiquitin and tau proteins. Motor neuron disease-type dementia is characterised by ubiquitin-immunoreactive intraneuronal inclusions in cortical layer II and the hippocampal dentate granule cells. A diagnosis of Alzheimer's disease changes is based upon the presence of neurofibrillary tangles, which immunostain with antibodies to tau and ubiquitin, and many associated neuritic plaques. Corticobasal degeneration is diagnosed by the presence of tau-immunoreactive, but ubiquitin-non-reactive intraneuronal inclusions in cortical layer II and the substantia nigra. Pick's disease is restricted to cases with tau- and ubiquitin-immunoreactive spherical cortical intraneuronal inclusions (Pick bodies), best seen in the hippocampal dentate gyrus and frontotemporal cortex. Dementia of frontal type is the preferred term for cases in which no intraneuronal inclusions are seen with antisera to tau and ubiquitin. A practical approach to the pathological diagnosis of frontotemporal dementia and the differential diagnosis of the five disorders using immunohistochemical studies is provided.

Synaptotagmin, a synaptic vesicle protein, is present in human cerebrospinal fluid: a new biochemical marker for synaptic pathology in Alzheimer disease?

Using a novel approach, including affinity chromatography, reversed-phase chromatography, and chemiluminescence immunoblotting, we have for the first time been able to demonstrate one of the small synaptic vesicle proteins, synaptotagmin I, in cerebrospinal fluid (CSF). Two other small synaptic vesicle proteins, rab3a and synaptophysin, were not detectable. The approximate molecular weight of CSF-synaptotagmin was 65 kDa, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Further characterization of CSF synaptotagmin by high-performance capillary electrophoresis (HPCE) showed a single peak. These findings support that the whole synaptotagmin molecule is present in CSF, without significant proteolytic degradation. After high-speed centrifugation of CSF, synaptotagmin was exclusively found in the supernatant, suggesting that synaptotagmin is present in CSF as a free protein, and not as a constituent of synaptic vesicles. In a preliminary study, we found a marked reduction of CSF synaptotagmin in patients with early onset Alzheimer disease (EAD) as compared with age-matched healthy individuals. To elucidate the biological relevance of this finding, we also quantified synaptotagmin in brain tissue. A marked reduction in synaptotagmin was found both in the hippocampus and frontal cortex of EAD, suggesting that a decrease in synaptotagmin in the brain is followed by a concomitant decrease in the CSF. Analysis of CSF synaptotagmin might provide a tool to study synaptic function and pathology in the human brain.

Correlations between mental state and quantitative neuropathology in the Vienna Longitudinal Study on Dementia

Quantitative clinicopathological correlation studies are one way to address the question of the relevance of morphological abnormalities in Alzheimer's dementia (AD). This paper summarizes results of the Vienna Longitudinal Study on Dementia obtained during the past few years and presents a critical discussion on the relevance of clinicopathological correlation studies for the pathogenesis of AD. Plotting of psychometric test scores against the numbers of plaques, tangles and neuropil threads in various cortical areas shows that significant correlations are due primarily to very high lesion counts in severely demented patients. These data indicate that neocortical neurofibrillary pathology can be considered an end-stage marker in the pathology of AD. On the other hand, the topographical staging of neuritic Alzheimer changes proposed by Braak and Braak (1991) appears to be a better reflection of the progression of the degenerative process than numerical lesion counts; there is a linear correlation between the Braak stages and Mini-Mental State scores in 122 aged individuals. Significant correlations are further obtained between the severity of dementia and the levels of a number of synaptic proteins including synaptophysin and the chromogranins. Taken together, our data suggest that none of the classical AD lesions, plaques and tangles, play a central role in the pathogenesis of dementia, a fact that is supported by a molecular biological study showing that there is no close relationship between these lesions and the neurons undergoing degeneration in AD. Whereas neuritic pathology is a useful histopathological marker for the diagnosis and staging of AD, the major correlate of cognitive deficits is the loss of corticocortical and subcorticocortical connections reflected by a depletion of synapses. This pathology may be induced by a mismetabolism of the beta-amyloid precursor proteins or their interaction with cytoskeletal proteins related to neuronal degeneration.

Structural basis of dementia in neurodegenerative disorders

Progressive dementia syndromes in adults are caused by a number of conditions associated with different structural lesions of the brain. In most clinical and autopsy series, senile dementia of the Alzheimer type is the most common cause of mental decline in the elderly accounting for up to 90%, whereas degenerative non-Alzheimer dementias range from 7 to 30% (mean 8-10%). They include a variety of disorders featured morphologically by neuron and synapse loss and gliosis, often associated with cytopathological changes involving specific cortical and subcortical circuits. These neuronal/glial inclusions and neuritic alterations show characteristic immunoreactions and ultrastructure indicating cytoskeletal mismetabolism. They are important diagnostic sign posts that, in addition to the distribution pattern of degenerative changes, indicate specific vulnerability of neuronal populations, but their pathogenic role and contribution to mental decline are still poorly understood. In some degenerative disorders no such cytopathological hallmarks have been observed; a small number is genetically determined. While in Alzheimer's disease (AD) mental decline is mainly related to synaptic and neuritic pathologies, other degenerative disorders show variable substrates of dementia involving different cortical and/or subcortical circuits which may or may not be superimposed by cortical Alzheimer lesions. In most demented patients with Lewy body disorders (Parkinson's disease, Lewy body dementia), they show similar distribution as in AD, while in Progressive Supranuclear Palsy (PSP), mainly prefrontal areas are involved. Lobar atrophies, increasingly apparent as causes of dementia, show fronto-temporal cortical neuron loss, spongiosis and gliosis with or without neuronal inclusions (Pick bodies) and ballooned cells, while dementing motor neuron disease and multisystem atrophies reveal ubiquitinated neuronal and oligodendroglial inclusions. There are overlaps or suggested relationships between some neurodegenerative disorders, e.g. between corticobasal degeneration, PSP and Pick's atrophy. In many of these disorders with involvement of the basal ganglia, degeneration of striatofrontal and hippocampo-cortical loops are important factors of mental decline which may be associated with isocortical neuronal degeneration and synapse loss or are superimposed by cortical AD pathology.

Neuropeptide changes in cortical and deep gray structures in Alzheimer's disease

The alteration of certain neuropeptide levels is a dramatic and consistent finding in the brains of AD patients. Levels of SS, which is normally present in high concentrations in cerebral cortex /75/, are consistently decreased in the neocortex, hippocampus and CSF of AD patients. In addition, decreased levels of SS correlate regionally with the distribution of neurofibrillary tangles in AD /47/. Most available evidence suggests that the subset of SS-containing neurons which lack NADPH diaphorase may be relatively vulnerable to degeneration in AD. CRF is another neuropeptide with frequently observed changes in AD. Levels of CRF, which is normally present in low concentrations in cortical structures /75/, are decreased in the neocortex and hippocampus of AD patients. However, levels of CRF in the CSF of AD patients are not consistently reduced, but this is likely a reflection of the relatively low levels of CRF normally present in cerebral cortex. Studies of deep gray structures in AD brains reveal elevated levels of GAL in the nucleus basalis. The ability of GAL to inhibit cholinergic neurotransmission has generated considerable interest, since degeneration of cholinergic neurons in the basal forebrain consistently occurs in AD. In addition, the presence of NADPH diaphorase in GAL-containing neurons may underlie the relative resistance of these neurons to degeneration. From the aforementioned studies, it appears that the neurons which are relatively resistant to neurodegeneration in AD contain NADPH diaphorase. It is hypothesized that the presence of NADPH diaphorase protects these neurons from neurotoxicity mediated by glutamate or nitric oxide. Although one recent study /147/ has reported an elevation of the microtubule-associated protein tau in the CSF of AD patients (and this could become a useful antemortem diagnostic tool for AD), no similar CSF abnormality has been found for any of the neuropeptides. Thus, the measurement of CSF neuropeptide levels presently remains unhelpful in the diagnosis and treatment of AD. Future research on neuropeptides and their potential roles in the pathogenesis, diagnosis, and treatment of AD will likely involve further development of pharmacological modulators of neuropeptide systems, together with the further study of brain neuropeptidases.

Clinical classification of dementias

The diagnosis of dementia disorders can be made in several steps. The symptomatology, severity and course of the illness must be considered. Memory disturbances must be present. Ante-mortem classification of dementias is based on known or assumed etiology. The final step is the post-mortem diagnosis. Of the primary dementias, Alzheimer-type dementia is the most important group. Accumulating data indicate that this form can be divided into subgroups. The concept of multi-infarct dementia (MID) should not be used synonymously with vascular dementia, which is a broader concept. Vascular dementias can also be divided into subgroups. Dementia is an advanced stage of a brain disorder. It seems important to identify brain disorders before they have reached the advanced level of dementia. A name for such disorders is dysmentia.

Secretogranin II: molecular properties, regulation of biosynthesis and processing to the neuropeptide secretoneurin

Secretogranin II is an acidic secretory protein in large dense core vesicles of endocrine, neuroendocrine and neuronal tissues. It comprises, together with chromogranins A and B, the class of proteins collectively called chromogranins. In this review the physico-chemical properties, genomic organization, tissue distribution, synthesis regulation, ontogeny and physiological function of this protein are discussed. Secretogranin II gained interest recently for mainly three reasons: (1) secretogranin II is an excellent marker for the regulated secretory pathway due to its simple and specific metabolic labeling by inorganic sulfate; (2) secretogranin II occurs in a variety of neoplasms arising from endocrine and neuroendocrine cells and was shown to be a useful histological tumor marker for these cells; (3) secretogranin II is the precursor of the recently discovered neuropeptide secretoneurin which induces dopamine release in the striatum of the rat brain.

Altered synaptophysin expression as a marker of synaptic pathology in schizophrenia

It has been proposed that synaptic density or synaptic innervation may be altered in schizophrenia as a correlate of the neurodevelopmental pathology of the disease. Synaptophysin is a synaptic vesicle protein whose distribution and abundance provides a synaptic marker which can be reliably measured in post mortem brain. We have used in situ hybridization histochemistry and immunoreactivity to assess the expression of synaptophysin messenger RNA and protein respectively in medial temporal lobe from seven schizophrenics and 13 controls. In the schizophrenic cases, synaptophysin messenger RNA was reduced bilaterally in CA4, CA3, subiculum and parahippocampal gyrus, with a similar trend in dentate gyrus but no change in CA1. It was also decreased in terms of grains per pyramidal neuron in the affected subfields. In parahippocampal gyrus, the loss of synaptophysin messenger RNA per neuron in schizophrenia was greater in deep than superficial laminae. A parallel study in rats showed no effect of haloperidol treatment upon hippocampal synaptophysin messenger RNA, suggesting that neuroleptic treatment does not underlie the reductions found in schizophrenia. In the right medial temporal lobe of schizophrenics, we confirmed the correlation of synaptophysin messenger RNA abundance between ipsilateral subfields seen in both hemispheres of control brains. However, these correlations were not observed in the left medial temporal lobe of the schizophrenic cases. Synaptophysin immunoreactivity in schizophrenia showed no significant differences in any subfield compared to controls. Our data support the broad hypothesis that synaptic pathology occurs in schizophrenia. In so far as synaptophysin expression is a marker for synaptic density, the data suggest that pyramidal neurons within the medial temporal lobe may form fewer synapses. However, the lack of any significant differences in synaptophysin immunoreactivity despite the loss of encoding messenger RNA means that this conclusion must be drawn cautiously. There are several plausible explanations for the preservation of synaptophysin immunoreactivity despite reductions in transcript abundance; one possibility is that the inferrred loss of synapses occurs in extra-hippocampal sites to which the affected pyramidal neurons project. For example, the reduction in synaptophysin messenger RNA in subicular neurons may be accompanied by decreased density of synaptic terminals in the nucleus accumbens. Such differences in the efferent synaptic connectivity of the hippocampus have previously been hypothesized to be an important component of the circuitry underlying schizophrenia.

5-HT1A agonist and dexamethasone reversal of para-chloroamphetamine induced loss of MAP-2 and synaptophysin immunoreactivity in adult rat brain

Serotonin and dexamethasone act as differentiating agents during development. Reducing circulating adrenal steroids or central 5-HT levels via adrenalectomy (ADX) or the tryptophan hydroxylase inhibitor, para-chlorophenylalanine (PCPA), respectively, has been shown to have de-differentiating effects in the adult brain. Morphometric analysis of 5-HT, S100 beta, MAP-2 and synaptophysin immunoreactivity (IR) was used to follow the molecular plasticity of several brain regions after lesioning of 5-HT nerve terminals by para-chloroamphetamine (PCA; 2 x 10 mg/kg s.c.), a serotonin neurotoxin. Two weeks after PCA treatment we observed reductions of 5-HT, S100 beta, and MAP-2 IR in parietal and temporal cortex, temporal pole, hippocampus and hypothalamus. The reductions in MAP-2 and synaptophysin-IR were reversed by 3 days of treatment with dexamethasone (10 mg/l drinking water) or ipsapirone, a 5-HT1A agonist (1 mg/kg s.c.). The loss of S100-IR was reversed only by the 5-HT1A agonist. These results indicate that both dexamethasone and serotonin have effects on adult neuronal plasticity but may work via different mechanisms. The implications of these findings to the loss of synaptophysin and MAP-2 staining in Alzheimer's disease are discussed.

Pick's disease immunohistochemistry: new alterations and Alzheimer's disease comparisons

Pick's disease (PD) brains were examined immunohistochemically for the expression of antigens known to be associated with Alzheimer's disease (AD) lesions. Most antibodies which label intracellular neurofibrillary tangles (NFTs) in AD were found to stain Pick bodies (PBs). Among them was the monoclonal antibody A2B5, which is known to recognize neuronal surface gangliosides. This result indicates that membrane proteins are probably incorporated into PBs as into NFTs. However, PBs, in contrast to NFTs, showed a paucity of staining for heparan sulfate glycosaminoglycan and basic fibroblast growth factor (bFGF). Staining for midkine, seen in senile plaques in AD, was not seen in PD. The relative lack of staining for these two neurotrophic factors in PD brain may reflect underlying mechanisms which are distinct from those in AD. We also describe two glial abnormalities in PD: glial fibrillary tangles and clusters of granules positive for the complement protein C4d in the hippocampal dentate fascia. These are presumably related to complement-activated oligodendroglia, and both pathological structures are more abundant in advanced cases, suggesting that they may be hallmarks of the disease progression.

The precursor protein of non-A beta component of Alzheimer's disease amyloid is a presynaptic protein of the central nervous system

Non-A beta component of Alzheimer's disease amyloid (NAC) is the second component in the amyloid from brain tissue of patients affected with Alzheimer's disease. Its precursor protein (NACP) was shown to be a brain-specific protein. In rat brain, NACP was more abundant in the neocortex, hippocampus, olfactory bulb, striatum, thalamus, and cerebellum and less abundant in the brain stem. Confocal laser microscopy analysis revealed that anti-NACP immunostaining was colocalized with synaptophysin-immunoreactive presynaptic terminals. Ultrastructural analysis showed that NACP immunoreactivity was associated with synaptic vesicles. NACP sequence showed 95% identity with that of rat synuclein 1, a synaptic/nuclear protein previously identified in rat brain, and good homology with Torpedo synuclein from the electric organ synapse and bovine phosphoneuroprotein 14 (PNP-14), a brain-specific protein present in synapses. Therefore, NACP is a synaptic protein, suggesting that synaptic aberration observed in senile plaques might be involved in amyloidogenesis in Alzheimer's disease.

Cell death in Alzheimer's disease evaluated by DNA fragmentation in situ

Loss of nerve cells is a hallmark of the pathology of Alzheimer's disease (AD), yet the patterns of cell death are unknown. By analyzing DNA fragmentation in situ we found evidence for cell death not only of nerve cells but also of oligodendrocytes and microglia in AD brains. In average, 30 times more brain cells showed DNA fragmentation in AD as compared to age-matched controls. Nuclear alterations suggestive of apoptosis were rare in degenerating cells. Even though the majority of degenerating cells were not located within amyloid deposits and did not contain neurofibrillary tangles, neurons situated within areas of amyloid deposits or affected by neurofibrillary degeneration revealed a higher risk of DNA fragmentation and death than cells not exposed to these AD changes.

Synapse alterations in the hippocampal-entorhinal formation in Alzheimer's disease with and without Lewy body disease

We quantified by microdensitometry the immunoreactivity (IR) to monoclonal antibodies (SP6, SP12, SP15 and SP18) against various synaptic proteins in the molecular layers of the dentate gyrus, CA4, CA3, CA1, subiculum and entorhinal cortex in Alzheimer's disease (AD), Lewy body variant of AD (LBV) and diffuse Lewy body disease (DLBD). A significant decrease in SP6 IR was observed in almost all regions in AD (28.4-70.1%, mean 41.3%), LBV (19.0-42.5%, mean 26.8%) and DLBD (19.9-31.7%, mean 27.1%) compared to controls. In addition, SP6 IR in the outer molecular layer of the dentate gyrus was strongly correlated with tangle count in the entorhinal cortex (r = -0.70, P < 0.002), suggesting loss of perforant pathway projection. Although the decrease in SP12 and SP15 IR was less pronounced, the mean values were decreased in dementia. Furthermore, SP12 and SP15 labeled a large number of neuritic plaques, and SP15 occasionally stained cortical LBs. The present findings indicate (i) that in the hippocampal-entorhinal formation, the decrease of synapse protein IR in AD is more severe than that in LBV and DLBD, (ii) that synaptic markers detect a subset of dystrophic neurites in the plaques and (iii) that synapse proteins are involved in the formation of cortical LBs.

Neurofibrillary tangle predominant form of senile dementia of Alzheimer type: a rare subtype in very old subjects

In a consecutive autopsy series of 580 demented elderly subjects, 256 with the clinical diagnosis of probable/possible Alzheimer's disease (AD), there were 10 cases aged between 80 and 99 years with moderate to severe dementia or confusional state in which neuropathological studies revealed abundant neurofibrillary tangles with predominant involvement of the allocortex (entorhinal region, subiculum, CA 1 sector of hippocampus, amygdala) but no or only very few senile plaques. Small numbers of diffuse deposits of beta A4 amyloid protein were present in the entorhinal cortex of 3 and in the isocortex of 5 brains, while neuritic plaques were totally absent. Only a few cases of this "senile dementia with tangles only" or, more correctly, "neurofibrillary predominant type of AD" corresponding to the limbic stage of neuritic AD pathology have been described in the literature. This rare subtype occurring in very old (over 80 years of age) subjects that does not fall within the currently used neuropathological criteria for diagnosis of AD warrants further clinico-pathological documentation.