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

Immunocytochemical localization of amyloid precursor protein in rat brain

The localization of amyloid precursor protein (APP) in rat brain was studied with a cytoplasmic domain-specific antibody. Light microscopic immunocytochemistry demonstrated that APP is present in most neurons, in some oligodendrocytes, and in a population of cells with diameters less than 10 microns that may be glial. Marked differences in immunoreactivity among neurons were observed, and the strongest immunoreactivity was contained in larger neurons. Neurons with scant cytoplasm, such as granule cells in the olfactory bulb, dentate gyrus, and cerebellum, were weakly immunoreactive. Differences in neuropil immunoreactivity were also observed; this type of staining was strongest in the caudatoputamen, lateral septum, medial habenula, nucleus reticularis of the dorsal thalamus, and the lateral portion of the ventroposterior nucleus. Neuropil immunostaining was weakest in layer IV of cortex and in areas containing granule cells. The fact that APP seems to be present in the vast majority of neurons suggests that this protein plays a role common to all neurons. The fact that there is a great difference in the steady-state amount of APP among different types of neurons suggests that APP may play a specific role in the function of certain classes of neurons.

Accumulation of amyloid precursor protein in brain lesions of patients with Pick disease

The expression of amyloid precursor protein (APP) was examined immunohistochemically in Pick disease brain using several antibodies to various segments of APP. Some neurons, including ballooned neurons, were intensely stained with antibodies to all but the N-terminal APP segment. However, Pick bodies were labeled with the antibody to that segment as well as the antibody to intermediate segment of APP. The pattern was similar to that previously observed for Alzheimer neurofibrillary tangles. These data provide additional evidence that Pick bodies and neurofibrillary tangles share some immunohistochemical characteristics.

Synaptophysin gene expression in human brain: a quantitative in situ hybridization and immunocytochemical study

Synaptophysin is a presynaptic vesicle protein. Its quantitative detection has become established as a molecular marker of synaptic density. We have studied synaptophysin messenger RNA in the neocortex, hippocampus and cerebellum using in situ hybridization histochemistry to see if the encoding transcript can be detected in post mortem human brain and to investigate factors which might influence its abundance. Synaptophysin was also measured immunocytochemically in the hippocampus. The level of synaptophysin messenger RNA expression was uniform in all neocortical areas examined. Strong correlations were found for the amount of synaptophysin messenger RNA between individual regions and between homologous areas in the two hemispheres. Synaptophysin messenger RNA declined with increasing age and prolonged post mortem interval. Synaptophysin immunoreactivity also reduced with age, as well as with duration of formalin fixation but not post mortem interval. Synaptophysin immunoreactivity correlated with the abundance of the messenger RNA in neurons within, and projecting to, each hippocampal subfield. Significantly greater synaptophysin immunoreactivity was seen in the left than the right CA4 and CA1 regions. These data show that quantitative detection of synaptophysin messenger RNA as well as synaptophysin itself can reliably be carried out in post mortem human brain sections. They are in keeping with other findings that synaptic density is relatively uniform through the neocortex and decreases with age. They also suggest a possible asymmetry of hippocampal synaptophysin expression. The level of synaptophysin messenger RNA paralleled that of synaptophysin immunoreactivity, indicating that changes in gene expression contribute to variations in the latter observed in aging and other situations. Detection of synaptophysin messenger RNA broadens the range of methods by which synaptic protein gene products can be studied and used as markers of synaptic density and synaptic involvement during physiological and pathological processes in human brain.

Expression of chromogranin A in lesions in the central nervous system from patients with neurological diseases

Expression of chromogranin A in various neurological diseases was examined immunohistochemically using purified anti-human chromogranin A antiserum. The antibody stained dystrophic neurites in senile plaques in Alzheimer disease brain, Pick bodies and ballooned neurons in Pick's disease brain, some Lewy bodies in the substantia nigra of Parkinson's disease, and axonal swellings in various neurological conditions including Parkinson's disease, striatonigral degeneration, Shy-Drager syndrome, amyotrophic lateral sclerosis and cerebral infarction. The present study shows that expression of chromogranin A is not an exclusive feature of Alzheimer disease or Pick's disease, and indicates that it could be a useful marker for various neurological diseases.

Membrane lipids, selectively diminished in Alzheimer brains, suggest synapse loss as a primary event in early-onset form (type I) and demyelination in late-onset form (type II)

Major membrane lipids were quantified in frontal (Brodmann area 9) and temporal (Brodmann areas 21 and 22) cortices, caudate nucleus, hippocampus, and frontal white matter of 12 cases with Alzheimer's disease (AD) type I (early onset), 21 cases with AD type II (late onset), and 20 age-matched controls. The concentration of gangliosides--a marker for axodendritic arborization--was reduced to 58-70% of the control concentration in all four gray areas (p < 0.0001) and to 81% in frontal white matter (p < 0.01) of AD type I cases, whereas it was only significantly reduced in temporal cortex (p < 0.01), hippocampus (p < 0.05), and frontal white matter (p < 0.05) in AD type II cases. The concentration of phospholipids was also significantly reduced (p < 0.01-0.0001) in all four gray areas of AD type I cases but in no area of AD type II cases. The loss of cholesterol was only 50% of the corresponding phospholipid diminution in AD type I. These results suggested a pronounced loss of nerve endings in AD type I. The characteristic membrane lipid disturbance in AD type II was a loss of myelin lipids. This is the first time a fundamental biochemical difference has been shown between the two major forms of AD.

Topographical distribution of synaptic-associated proteins in the neuritic plaques of Alzheimer's disease hippocampus

Studies of the molecular composition of the abnormal neuritic processes of the plaques in Alzheimer's disease (AD) have shown that these structures are immunoreactive with antibodies against growth-related molecules, synaptic/axonal proteins, and cytoskeletal proteins. These studies suggest that a subpopulation of abnormal neurites in the plaque are sprouting axons that eventually degenerate. To test this hypothesis further we studied the regional distribution of plaques in the hippocampus using a panel of monoclonal antibodies against synaptic proteins. With these antibodies we found a greater proportion of immunoreactive plaques compared to previous studies where a monoclonal antibody against synaptophysin was used. The most sensitive antibodies to detect neuritic plaques were SP11 and anti-p65, and the largest number of positive plaques was found in the entorhinal cortex and CA1 region. These results further support the theory that synaptic and axonal damage are involved in plaque formation in AD.

AF64A-induced brain damage and its relation to dementia

Several data obtained in the AF64A-model are of particular relevance for our understanding of the pathogenesis and progression of Alzheimer's disease. The AF64A-induced withdrawal of cholinergic function in the rat hippocampus was associated with reversible functional changes in other neurotransmitters, including noradrenaline, serotonin, somatostatin and glutamate, thereby mimicking changes in Alzheimer's disease. Identical changes in markers for synaptic vesicles were found in Alzheimer's disease and AF64A-model. A study on the role of gender revealed a higher susceptibility to the neurotoxic action of AF64A in female rats. The cholinergic deficit was also responsible for a disinhibition of the negative feedback regulation of glucocorticoids. Increased exposure to glucocorticoids, however, enhanced the vulnerability of hippocampal cholinergic neurons to AF64A. These data indicate that the AF64A-induced cholinergic deficit in the rat brain represents a reliable tool to study several mechanisms possibly involved in Alzheimer's disease.

Neuropathological staging of Alzheimer lesions and intellectual status in Alzheimer's and Parkinson's disease patients

In both Alzheimer's disease (AD) and Parkinson's disease (PD), neurofibrillary tangles (NFT), in contrast to amyloid deposits, show a hierarchical spreading pattern from the allocortex to isocortical association areas with early involvement of the entorhinal region, a major relay station between hippocampus and isocortex. Based on the distribution pattern of NFT in human brain, a neuropathological staging of neuritic AD pathology has been proposed. Comparative studies of this neuropathological staging of neuritic AD changes with psychometrically assessed intellectual status (mini-mental state) in prospective cohorts of 29 aged individuals and 28 PD patients showed a linear correlation of morphological AD staging with the psychostatus in both disorders. The pattern of neuronal degeneration associated with neuritic AD pathology in both AD and PD may be an important basis of cognitive decline in both disorders.

Synaptic pathology of Alzheimer's disease

Prospective clinico-pathological studies on dementia in Alzheimer's disease (AD), performed during the past decades, revealed a relatively poor correlation between the degree of clinical deficit and the severity of the typical neuropathological lesions of AD, the amyloid plaques and the neurofibrillary tangles. More recent data, obtained by electron microscopy, immunocytochemical as well as immunochemical techniques indicate that synaptic loss may be a better structural correlate of dementia than other brain lesions. Synaptic pathology is reflected by a loss of all major components of small synaptic vesicles and most peptides, stored in large dense cored vesicles. The significant increase of chromogranin A proprotein, a major component of large dense cored vesicles, may rather represent a defect of protein processing than preservation of a specific synaptic subpopulation. Within the brain of AD patients, the degree of synaptic loss is uneven. Most prominent reduction of synapses is found in the outer parts of the dentate gyrus molecular layer, possibly reflecting the destruction of neurons, located in the layer 2 of the entorhinal cortex. However, within the neocortex, no preferential loss of synapses in any of the cortical layers has been found. Cerebral amyloid deposition in diffuse plaques has little effect on synapse density and structure. However, within the dense amyloid core of a classical plaque, synapses are completely lost. In the surrounding neuritic portion of the plaques, synaptophysin reactivity is frequently increased, due to enlargement of synaptic boutons and to accumulation of synaptophysin in dystrophic axons. Although the reason for synapse loss in AD is yet unknown, most results suggest that it may reflect degeneration of neurons, projecting into the respective cortical areas.

Differential incorporation of processes derived from different classes of neurons into senile plaques in Alzheimer's disease

The incorporation of neurites into amyloid deposits is an important step in the formation of senile plaques in Alzheimer's disease. It is unknown whether all neuronal types contribute neurites equally to plaques, or whether the processes of certain types are preferentially incorporated. We addressed this question by comparing the incorporation into neocortical plaques of neurites containing the widely distributed neuronal markers chromogranin A (CgA), parvalbumin (PV) and calbindin D-28K (CaBP) in relation to the number of neuronal perikarya expressing each of these substances in the neocortex. We found a consistent, statistically significant ranking, so that CgA-immunoreactive (ir) neurites were preferentially incorporated into plaques in comparison with PV-ir, and PV-ir were favoured over CaBP-ir neurites.

Evidence that transmitter-containing dystrophic neurites precede those containing paired helical filaments within senile plaques in the entorhinal cortex of nondemented elderly and Alzheimer's disease patients

Within the amygdala of elderly subjects and patients with Alzheimer's disease (AD), we recently found evidence suggesting amyloid beta-protein (A beta P) deposition occurs before the appearance of dystrophic neurites. Moreover, these data suggested dystrophic neurites initially lack evidence of cytoskeletal pathology although with time and further maturation, the dystrophic neurites display an altered cytoskeleton as evidenced by their immunoreactivity to Alz-50 and paired-helical filaments (PHF). These findings are of particular relevance to our understanding of the sequence of pathologic events in AD and thus it has become important to determine whether these events are unique to the amygdala or are representative of a more general pattern which can be found throughout the brain. Using a battery of antibodies to markers that are characteristic of AD pathology (i.e., A beta P, PHF, and Alz-50), three peptidergic neurotransmitters (neurotensin, somatostatin, and substance P), and one neurotransmitter biosynthetic enzyme (choline acetyltransferase), we examined the entorhinal cortex (EC) of three groups of subjects (AD, normal elderly, and a group of nondemented elderly with numerous senile plaques). The EC was studied, in part, because it is well recognized as a brain region displaying severe and, most importantly, early pathologic changes. Like the amygdala, we found evidence that amyloid beta-protein immunoreactive (A beta P-IR) and thioflavine-S-positive senile plaques occur within the EC prior to the appearance of transmitter-, Alz-50-, or PHF-immunoreactive dystrophic neurites. We also observed transmitter-immunoreactive dystrophic neurites in the absence of Alz-50 or PHF-immunolabeled dystrophic neurites and transmitter- and Alz-50-IR dystrophic neurites in the absence of those containing PHF. Collectively, these findings were similar to those seen within the amygdala and thus reinforced the concept that A beta P deposition is the primary event in plaque pathology, and this deposition is subsequently followed by the appearance of dystrophic neurites which retain their transmitter phenotype yet lack an altered cytoskeleton. With time, these dystrophic neurites develop cytoskeletal alterations and become immunoreactive to Alz-50 and PHF.

Vascular dementia in Spatz-Lindenberg's disease (SLD): cortical synaptophysin immunoreactivity as compared with dementia of Alzheimer type and non-demented controls

The generalized form of von Winiwarter-Buerger's disease (WBD) occasionally involves the brain. However, pure cerebral forms of the disease were also described by Spatz and Lindenberg ("Spatz-Lindenberg's disease", SLD). Both, the type I, which involves the large basal arteries, and the type II, which results in a sickle-shaped granular atrophy of the cerebral cortex, are often accompanied by ("vascular") dementia, which Lindenberg and Spatz mainly attributed to the bilateral involvement of the second frontal gyrus by granular atrophy. Recently, synaptic deprivation of the cortical gray matter has been shown to occur in the dementia of Alzheimer type (DAT) and other neurodegenerative disorders. In DAT, the synaptic loss highly correlated with the degree of the mental impairment. We wanted to examine whether similar changes also occurred in dementia of vascular origin, for which SLD, although infrequent, is a typical example. In fact, we found that in three cases of typical SLD type II the synaptophysin immunoreactivity of the cortical neuropil in areas without overt infarcts or scar formation was as much reduced as in Alzheimer's disease. Although it must be taken into account that in the present cases the synapse loss might, at least in part, be due to secondary (Wallerian) degeneration as a result of the neuronal loss in the "watershed" regions of the arterial blood supply, it cannot be excluded that a decline of cortical synaptic contacts in areas without necroses or scars may occur as a primary event, contributing to the pathogenesis of the dementia. Final conclusions can only be expected from investigations into further cases of cerebro-vascular disorders with and without dementia.

Cerebrospinal fluid chromogranin A is unchanged in Alzheimer dementia

Several lines of evidence link chromogranin A (CgA), the major soluble protein in catecholamine storage vesicles, with the cholinergic nervous system, abnormalities of which may play a central role in memory deficits in Alzheimer dementia. Because of reported elevations of CgA in Alzheimer brains and its presence in the senile plaque lesions of such brains, we evaluated the concentration of CgA in cerebrospinal fluid of Alzheimer dementia patients and matched controls. CgA was detectable in each sample, but the results in dementia showed substantial overlap with and no significant (p = 0.55) difference from the results in healthy controls. We conclude that measurement of cerebrospinal fluid CgA offers no diagnostic assistance in Alzheimer dementia.

Posttranslational processing of proenkephalins and chromogranins/secretogranins

Posttranslational processing of peptide-precursors is nowadays believed to play an important role in the functioning of neurons and endocrine cells. Both proenkephalins and chromogranins/secretogranins are considered as precursor molecules in these tissues, resulting in posttranslationally formed degradation products with potential biological activities. Among the proteins and peptides of neuronal and endocrine secretory granules, the enkephalins and enkephalin-containing peptides have been most extensively studied. The characterization of the post-translationally formed degradation products of the proenkephalins have enabled the understanding of their processing pathway. Chromogranins/secretogranins represent a group of acidic glycoproteins, contained within hormone storage granules. The biochemistry, biogenesis and molecular properties of these proteins have already been studied for 25 years. The chromogranins/secretogranins have a widespread distribution throughout the neuroendocrine system, the adrenal medullary chromaffin granules being the major source of these storage components. Recent data provide evidence for a precursor role for all members of the chromogranins/secretogranins family although also several other functions have been proposed. In this review, some of the methods applied to study proteolytic processing are described. In addition, the posttranslational processing of chromogranins/secretogranins and proenkephalins, especially the biochemical aspects, will be discussed and compared. Recent exciting developments on the generation and identification of potential physiologically active fragments will be covered.

The role of synaptic proteins in the pathogenesis of disorders of the central nervous system

Complex sets of nervous system functions are dependent on proper working of the synaptic apparatus, and these functions are regulated by diverse synaptic proteins that are distributed in various subcellular compartments of the synapse. The most extensively studied synaptic proteins are synaptophysin, the synapsins, growth associated protein 43 (GAP-43), SV-2, and p65. Moreover, synaptic terminals contain a great number of other proteins involved in calcium transport, neurotransmission, signaling, growth and plasticity. Probes against various synaptic proteins have recently been used to study synaptic alterations in human disease, as well as in experimental models of neurological disorders. Such probes are useful markers of synaptic function and synaptic population density in the nervous system. For the present, we will review the role of synaptic proteins in the following conditions: Alzheimer's disease (AD) and other disorders including ischemia, disorders where synapse-associated proteins are abnormally accumulated in the nerve terminals, synaptic proteins altered after denervation, and synaptic proteins as markers in neoplastic disorders. The study of the molecular alterations of the synapses and of plasticity might yield important clues as to the mechanisms of neurodegeneration in AD, and of the patterns of presynaptic and dendritic damage under diverse pathological conditions.