Relationship between pigment accumulation and age in Alzheimer's disease and Down syndrome

Alterations of Neuronal Lysosomes in Alzheimer's Disease and in APPxPS1-KI Mice

BACKGROUND

The cellular and molecular alterations associated with synapse and neuron loss in Alzheimer's disease (AD) remain unclear. In transgenic mouse models that express mutations responsible for familial AD, neuronal and synaptic losses occur in populations that accumulate fibrillar amyloid-β 42 (Aβ 42) intracellularly.

OBJECTIVE

We aimed to study the subcellular localization of these fibrillar accumulations and whether such intraneuronal assemblies could be observed in the human pathology.

METHODS

We used immunolabeling and various electron microscopy techniques on APP x presenilin1 - knock-in mice and on human cortical biopsies and postmortem samples.

RESULTS

We found an accumulation of Aβ fibrils in lipofuscin granule-like organelles in APP x presenilin1 - knock-in mice. Electron microscopy of human cortical biopsies also showed an accumulation of undigested material in enlarged lipofuscin granules in neurons from AD compared to age-matched non-AD patients. However, in those biopsies or in postmortem samples we could not detect intraneuronal accumulations of Aβ fibrils, neither in the lipofuscin granules nor in other intraneuronal compartments.

CONCLUSION

The intralysosomal accumulation of Aβ fibrils in specific neuronal populations in APPxPS1-KI mice likely results from a high concentration of Aβ 42 in the endosome-lysosome system due to the high expression of the transgene in these neurons.

Perineuronal nets potentially protect against oxidative stress

A specialized form of extracellular matrix (ECM) termed perineuronal nets (PNs) consisting of large aggregating chondroitin sulfate proteoglycans (CSPGs), with hyaluronan and tenascin as main components, surrounds subpopulations of neurons. The glycosaminoglycan components of perineuronal nets form highly charged structures in the direct microenvironment of neurons and thus might be involved in local ion homeostasis. The polyanionic character suggests that perineuronal nets also potentially contribute to reduce the local oxidative potential in the neuronal microenvironment by scavenging and binding redox-active iron, thus providing some neuroprotection to net-associated neurons. Here, we show that neurons ensheathed by a perineuronal net in the human cerebral cortex are less frequently affected by lipofuscin accumulation than neurons without a net both in normal-aged brain and Alzheimer's disease (AD). As lipofuscin is an intralysosomal pigment composed of cross-linked proteins and lipids generated by iron-catalyzed oxidative processes, the present results suggest a neuroprotective function of perineuronal nets against oxidative stress, potentially involved in neurodegeneration.

Functional and morphological alterations in compound transgenic mice overexpreszing Cu/Zn superoxide dismutaze and amyloid precursor protein

Down's syndrome (DS), the phenotypic manifestation of trisomy 21, involves overexpression of chromosome 21-encoded genes. The gene for amyloid precursor protein (APP), known to be involved in AD pathology, resides on chromosome 21 along with the gene for Cu/Zn superoxide dismutase (SOD1), a key enzyme in the metabolism of oxygen free radicals. We investigated the consequences of a combined increase in APP and SOD1, in a double-transgenic (tg)-APP-SOD1 mouse. These mice expressed severe impairment in learning, working and long-term memory. Expression of long-term potentiation in hippocampal slices was impaired in both tg-SOD and tg-APP-SOD mice, but not in tg-APP mice, indicating that increased APP by itself did not affect in vitro synaptic plasticity. In tg-APP-SOD mice, membrane-bound high molecular weight APP species accumulated while APP cleavage products did not increase and levels of secreted APP were unchanged. Severe morphological damage, including lipofuscin accumulation and mitochondria abnormalities, were found in aged tg-APP-SOD but not in the other mice. Thus, a combined elevation of the two chromosome 21 genes in tg-APP-SOD mice induced age-dependent alterations in morphological and behavioural functions.

Gene expression and cellular content of cathepsin D in Alzheimer's disease brain: evidence for early up-regulation of the endosomal-lysosomal system

In Alzheimer's disease brains, more than 90% of pyramidal neurons in lamina V and 70% in lamina III displayed 2- to 5-fold elevated levels of cathepsin D (Cat D) mRNA by in situ hybridization compared with neurologically normal controls. Most of these cells appeared histologically normal. The less vulnerable nonpyramidal neuron population in lamina IV had relatively normal message levels. Neuronal populations expressing more Cat D mRNA also displayed quantitatively increased Cat D immunoreactive protein. Cat D mRNA expression was only moderately increased in astrocytes. Degenerating neurons exhibited intense immunoreactivity but lowered Cat D mRNA levels. The upregulation of Cat D synthesis and accumulation of hydrolase-laden lysosomes indicate an early activation of the endosomal-lysosomal system in vulnerable neuronal populations, possibly reflecting early regenerative or repair processes. These abnormalities also represent a basis for altered regulation of amyloid precursor protein processing.

Differences in the pattern of hippocampal neuronal loss in normal ageing and Alzheimer's disease

The distinction between the neurodegenerative changes that accompany normal ageing and those that characterise Alzheimer's disease is not clear. The resolution of this issue has important implications for the design of therapeutic and investigative strategies. To this end we have used modern stereological techniques to compare the regional pattern of neuronal cell loss in the hippocampus related to normal ageing to that associated with Alzheimer's disease. The loss related to normal ageing was evaluated from estimates of the total number of neurons in each of the major hippocampal subdivisions of 45 normal ageing subjects who ranged in age from 13 to 101 years. The Alzheimer's disease related losses were evaluated from similar data obtained from 7 cases of Alzheimer's disease and 14 age matched controls. Qualitative differences were observed in the regional patterns of neuronal loss related to normal ageing and Alzheimer's disease. The most distinctive Alzheimer's disease related neuron loss was seen in the CA1 region of the hippocampus. In the normal ageing group there was almost no neuron loss in this region (final neuron count in the CA1 region: 4.40 x 10(6) neurons for the Alzheimer's disease group vs 14.08 x 10(6) neurons in the normal ageing group). It is concluded that the neurodegenerative processes associated with normal ageing and with Alzheimer's disease are qualitatively different and that Alzheimer's disease is not accelerated by ageing but is a distinct pathological process.

Monoamine neurons in aging and Alzheimer's disease

The integrity of dopaminergic, noradrenergic and serotonergic neurons in normal aging and Alzheimer's disease is reviewed. Loss of dopaminergic innervation of the neostriatum is a prominent age-related change, which corresponds with the age-related loss of dopaminergic cell bodies from the substantia nigra. This change is regionally specific, since dopaminergic innervation of the neocortex and the neostriatum are not affected. Although there is an age-related loss of noradrenergic cell bodies from the locus coeruleus, most studies indicate normal concentrations of noradrenaline in target areas. There is also evidence for reduced serotonergic innervation of the neocortex and, less convincingly, the neostriatum. Alzheimer's disease is associated with more pronounced noradrenergic and serotonergic denervation but, unlike normal aging, dopaminergic innervation of neostriatum is intact; although dopamine neurons are probably dysfunctional in this region. Studies relating neuronal markers to the symptomatology of Alzheimer's disease indicate that dysfunction of monoamine neurons is more closely linked to non-cognitive than to cognitive changes in behavior. In addition, monoaminergic therapies have been successful in ameliorating affective and psychotic behaviors along with sleep disturbances in both Alzheimer's disease and senescence. It seems likely that monoaminergic therapies (developed as we learn more about alterations in dopamine, noradrenaline and serotonin) will continue to be necessary to treat such behavioral disturbances.

Involvement of free radicals in dementia of the Alzheimer type: a hypothesis

We propose that increased formation of oxygen-derived free radicals, such as the superoxide and hydroxyl species, may be responsible for progressive neural degeneration in dementia of the Alzheimer type (DAT). Several processes increase free radical formation and some of them (e.g., brain trauma, aging) are risk factors for DAT. There is some evidence for increased free radical formation in Down's syndrome which is associated with development of DAT pathology. Free radicals alone may induce cell death by damaging lipids or proteins while reactions between free radicals and neurotransmitters may lead to formation of endogenous neurotoxin(s). Recently, we have demonstrated that partial oxidation of serotonin by exposure to hydroxyl radicals results in formation of a novel neurotoxin, tryptamine-4,5-dione. Elucidation of the role of free radicals in DAT could open new avenues to prevention and treatment of this disease.

Lipid abnormalities in the brain in adult Down's syndrome and Alzheimer's disease

Quantitative analysis by HPTLC of the major lipid classes and dolichol, and of fatty acyl groups of separated phosphoglycerides by capillary GLC, has been carried out on the gray matter of frontal cerebral cortex of brains from six Down's syndrome (DS) and six Alzheimer's disease (AD) adults, and six each of two corresponding sets of age-matched controls; specimens of DS and control cerebellum and corpus callosum were also analyzed. In DS frontal cortex, but not in AD frontal cortex, compared to their respective controls there was a decrease in the fraction of phosphatidylethanolamine (PE) and an increase in the fractions of sphingomyelin (SPM) and phosphatidylserine (PS). Abnormalities were not found in the proportions of major lipid classes in DS cerebellum or corpus callosum. The concentration of dolichol was elevated for age in the frontal cortex of DS and of AD. In the phosphoglycerides of DS frontal cortex, the fatty acyl composition showed small, but statistically significant, differences from those of age-matched controls, and some slight abnormalities were also detected in DS corpus callosum. The alterations in DS frontal cortex included decreases in (n-6) and increases in (n-3) groups in choline and ethanolamine phosphoglycerides (CPG and EPG), as had previously been found in EPG and serine phosphoglyceride (SPG) of the DS fetal brain. In DS frontal cortex, the proportion of 22:4(n-6) groups was decreased in SPG, and in inositol phosphoglyceride (IPG) 18:1(n-9) was increased. There were also small but significant alterations in DS frontal cortex in the fractions of shorter chain groups in CPG. In marked contrast, most of the fatty acyl abnormalities seen in DS were absent in the AD frontal cortex. It is therefore suggested that some abnormalities in the composition of cerebral membranes present prenatally in DS may persist into adulthood, and are not directly related to AD-type pathology.

The topography of nerve cell loss from the locus caeruleus in elderly persons

A topographical analysis of nerve cell loss from the locus caeruleus in "mentally normal" old people shows cell loss to be uniformly diffuse throughout the whole nucleus with no preferential involvement of any one particular area. Such findings contrast with those of ours on Alzheimer's disease and suggest differing mechanisms underlying the cell loss of old age and Alzheimer's disease. Cell loss in Alzheimer's disease is thought to relate to primary pathogenetic events in terminal fields of cerebral cortex. In "normal" old age, cell loss may be determined by changes occurring at perikaryal level possibly in respect of the cytotoxic effects of noradrenaline degradation and neuromelanin accumulation.

The pathological association between Down syndrome and Alzheimer disease

The neuropathology of Down syndrome (DS) at middle age is compared with that of Alzheimer disease (AD) at that age, through a review of the published literature and from the author's personal observations on brains from a series of patients of different ages with DS. It is noted that the pathological changes of DS at middle age (i.e. the form and distribution of senile plaques and neurofibrillary tangles, the pattern of involvement (atrophy) of neuronal systems) are qualitatively the same as those of AD at that age, though quantitative differences do occur and these may relate to biological or sociological variations inherent to the two parent populations. It is concluded that in pathological terms patients with DS at middle age do indeed have AD. Some ways in which a study of patients with DS can give insight into the nature and development of the pathological changes of AD are put forward and discussed.

Microspectrofluorometric quantitation of autofluorescent lipopigment in the human sympathetic ganglia

The age-related changes in lipopigment autofluorescence were studied by microspectrofluorometry in three different types of human neurons: the sympathetic neurons of the stellate and superior mesenteric ganglion and pyramidal neurons of the frontal cortex. The age-related increase in lipopigment autofluorescence was more rapid in stellate ganglion but similar linear increases were found also in superior mesenteric ganglion and frontal cortex. There was an age-related shift in the autofluorescence from yellow to orange in the ganglia. This may be due to the accumulation of neuromelanin in noradrenergic neurons. Lipopigments were identified in sympathetic neurons at the age of 4 months and all neurons carried pigment granules after the age of 64 years. It is concluded that lipopigment autofluorescence is a useful marker for cellular ageing in both the peripheral and the central nervous system.

The neuropathology of Alzheimer's disease: a review with pathogenetic, aetiological and therapeutic considerations

The neuropathology of Alzheimer's disease is reviewed in this paper emphasizing the morphological and morphometric changes that occur in the disease and their relationship to age and ageing. From this, a new hypothesis of pathogenesis is presented which accounts for the pattern of neuronal damage in Alzheimer's disease. This is that the pathogenesis of Alzheimer's disease begins with a leakage of a neurotoxin through a defective cortical blood brain barrier. This incites development of the senile plaque and later, via a retrograde transport of the same (or different) factors, intracellular neurofibrillary tangle formation and death of neurones within areas of cortex affected by plaques and in subcortical areas such as nucleus basalis of Meynert, locus caeruleus and dorsal raphe nuclei, all of which project to these same areas of cortex. Evidence consistent with this hypothesis is presented and the aetiological and therapeutic implications are discussed.