Neuroanatomical connections and specific regional vulnerability in Alzheimer's disease

Tau Prion-Like Propagation: State of the Art and Current Challenges

It has been almost a decade since the hypothesis of active tau protein propagation in Alzheimer's disease and associated tauopathies was formally raised. We view tau propagation as a cascade of events, starting with early tau misfolding, followed by transfer to another, anatomically connected, cell, contaminating in corruption of endogenous tau in the recipient cell through a seeding mechanism of templated misfolding. These mechanisms are very similar to those of other proteinopathies and to ideas about how prion pathologies spread through the brain. Nonetheless, the specific mechanisms underlying each of these steps remains uncertain and is a fertile ground for new experimental approaches potentially requiring new experimental models. We review, here, the state of the art of the research on tau prion-like propagation and we highlight some key challenges to understanding the detailed mechanisms of cell to cell propagation.

Alzheimer's Disease: Characterization of the Brain Sites of the Initial Tau Cytoskeletal Pathology Will Improve the Success of Novel Immunological Anti-Tau Treatment Approaches

Alzheimer's disease (AD) represents the most frequent neurodegenerative disease of the human brain worldwide. Currently practiced treatment strategies for AD only include some less effective symptomatic therapeutic interventions, which unable to counteract the disease course of AD. New therapeutic attempts aimed to prevent, reduce, or remove the extracellular depositions of the amyloid-β protein did not elicit beneficial effects on cognitive deficits or functional decline of AD. In view of the failure of these amyloid-β-based therapeutic trials and the close correlation between the brain pathology of the cytoskeletal tau protein and clinical AD symptoms, therapeutic attention has since shifted to the tau cytoskeletal protein as a novel drug target. The abnormal hyperphosphorylation and intraneuronal aggregation of this protein are early events in the evolution of the AD-related neurofibrillary pathology, and the brain spread of the AD-related tau aggregation pathology may possibly follow a corruptive protein templating and seeding-like mechanism according to the prion hypothesis. Accordingly, immunotherapeutic targeting of the tau aggregation pathology during the very early pre-tangle phase is currently considered to represent an effective and promising therapeutic approach for AD. Recent studies have shown that the initial immunoreactive tau aggregation pathology already prevails in several subcortical regions in the absence of any cytoskeletal changes in the cerebral cortex. Thus, it may be hypothesized that the subcortical brain regions represent the "port of entry" for the pathogenetic agent from which the disease ascends anterogradely as an "interconnectivity pathology".

Hierarchical Distribution of the Tau Cytoskeletal Pathology in the Thalamus of Alzheimer's Disease Patients

In spite of considerable progress in neuropathological research on Alzheimer's disease (AD), knowledge regarding the exact pathoanatomical distribution of the tau cytoskeletal pathology in the thalamus of AD patients in the advanced Braak and Braak AD stages V or VI of the cortical cytoskeletal pathology is still fragmentary. Investigation of serial 100 μm-thick brain tissue sections through the thalamus of clinically diagnosed AD patients with Braak and Braak AD stage V or VI cytoskeletal pathologies immunostained with the anti-tau AT8 antibody, along with the affection of the extraterritorial reticular nucleus of the thalamus, reveals a consistent and severe tau immunoreactive cytoskeletal pathology in the limbic nuclei of the thalamus (e.g., paraventricular, anterodorsal and laterodorsal nuclei, limitans-suprageniculate complex). The thalamic nuclei integrated into the associative networks of the human brain (e.g., ventral anterior and mediodorsal nuclei) are only mildly affected, while its motor precerebellar (ventral lateral nucleus) and sensory nuclei (e.g., lateral and medial geniculate bodies, ventral posterior medial and lateral nuclei, parvocellular part of the ventral posterior medial nucleus) are more or less spared. The highly stereotypical and characteristic thalamic distribution pattern of the AD-related tau cytoskeletal pathology represents an anatomical mirror of the hierarchical topographic distribution of the cytoskeletal pathology in the interconnected regions of the cerebral cortex of AD patients. These pathoanatomical parallels support the pathophysiological concept of a transneuronal spread of the disease process of AD along anatomical pathways. The AD-related tau cytoskeletal pathology in the thalamus most likely contributes substantially to the neuropsychiatric disease symptoms (e.g., dementia), attention deficits, oculomotor dysfunctions, altered non-discriminative aspects of pain experience of AD patients, and the disruption of their waking and sleeping patterns.

Precortical Phase of Alzheimer's Disease (AD)-Related Tau Cytoskeletal Pathology

Alzheimer's disease (AD) represents the most frequent progressive neuropsychiatric disorder worldwide leading to dementia. We systematically investigated the presence and extent of the AD-related cytoskeletal pathology in serial thick tissue sections through all subcortical brain nuclei that send efferent projections to the transentorhinal and entorhinal regions in three individuals with Braak and Braak AD stage 0 cortical cytoskeletal pathology and fourteen individuals with Braak and Braak AD stage I cortical cytoskeletal pathology by means of immunostainings with the anti-tau antibody AT8. These investigations revealed consistent AT8 immunoreactive tau cytoskeletal pathology in a subset of these subcortical nuclei in the Braak and Braak AD stage 0 individuals and in all of these subcortical nuclei in the Braak and Braak AD stage I individuals. The widespread affection of the subcortical nuclei in Braak and Braak AD stage I shows that the extent of the early subcortical tau cytoskeletal pathology has been considerably underestimated previously. In addition, our novel findings support the concept that subcortical nuclei become already affected during an early 'pre-cortical' evolutional phase before the first AD-related cytoskeletal changes occur in the mediobasal temporal lobe (i.e. allocortical transentorhinal and entorhinal regions). The very early involved subcortical brain regions may represent the origin of the AD-related tau cytoskeletal pathology, from where the neuronal cytoskeletal pathology takes an ascending course toward the secondarily affected allocortex and spreads transneuronally along anatomical pathways in predictable sequences.

Elevation of LDL receptor-related protein levels via ligand interactions in Alzheimer disease and in vitro

The low-density lipoprotein (LDL) receptor-related protein (LRP) is a multifunctional receptor in the CNS that binds both apolipoprotein E (apoE) and activated alpha2-macroglobulin (alpha2M*); all 3 proteins are genetically associated with Alzheimer disease (AD). In this study we found an 85% increase in LRP levels in human AD brain frontal cortex, along with an increased level of the LRP ligands, apoE, and alpha2M. We speculated that LRP levels might be increased in response to the increased levels of its ligands, apoE, and alpha2M*. To test this hypothesis we examined the effects of alpha2M* on LRP in primary cultures. Treatment of neurons with alpha2M* significantly increased LRP levels (by 92%). This increase was prevented by coculture with receptor-associated protein (RAP), which blocks binding of LRP ligands to LRP Native alpha2M or RAP alone did not change LRP levels in vitro. We also found that alpha2M* stimulated activation of astrocytes in vitro and promoted the levels of LRP by 65%. These data indicate 1) the LRP ligand alpha2M* increases levels of LRP in primary neuronal and astrocytic cultures, 2) alpha2M*-induction of LRP levels in vitro depends on binding to LRP, and 3) LRP levels are increased in AD brain, perhaps in response to the increased levels of alpha2M.

Mild amyloid pathology in the primary visual system of nonagenarians and centenarians

In order to study the patterns of Alzheimer disease (AD)-related pathology in the primary visual system of the oldest old, we performed a quantitative analysis of senile plaques (SP), diffuse beta amyloid (A beta) deposit and neurofibrillary tangle (NFT) distribution in primary area 17, and a semi-quantitative analysis in the dorsal lateral geniculate nucleus (LGN), lateral inferior pulvinar (LIP) and superior colliculus (SC) of 21 individuals aged between 93 and 102 years. Among them, 10 cases were considered as non-demented (ND), while 9 presented very mild cognitive impairment (VMCI), and 2 cases had a clinical diagnosis of AD. Silver methenamine and Gallyas staining, A beta and tau immunostaining revealed the distribution of AD lesions. In primary area 17, most cases, either ND or with VMCI displayed a low to medium number of SP. There was no significant difference in SP and A beta deposit densities between ND and VMCI groups. On the whole, 0.4--2.4% of the cross-sectional cortical area was covered with A beta deposits. Only 6 cases, either ND or with VMCI, were totally devoid of SP and diffuse A beta deposits. Among the subcortical structures, the LIP and SC exhibited low densities of SP and A beta deposits in about half of the ND and VMCI cases, while the LGN was totally spared. NFT were almost absent in area 17 and subcortical nuclei of ND and VMCI cases. These data imply that the ageing of the primary visual system in ND and VMCI nonagenarians and centenarians is characterised by the frequent development of mild amyloid pathology in area 17 in the absence of NFT. In agreement with previous studies in very old cohorts, they also suggest that amyloid deposition is not related to the early stages of the dementia process in the oldest old.

The evolution of Alzheimer's disease-related cytoskeletal pathology in the human raphe nuclei

The cross-sectional analyses currently available show that the Alzheimer's disease (AD)-related cytoskeletal alterations within the human brain affect variously susceptible areas of the cerebral cortex in a uniform sequence with very little interpatient variability. This sequence has been divided for research and comparative purposes into six stages (cortical NFT/NT-stages I-VI). Among the subcortical nuclei affected in AD are those belonging to the raphe system. Efforts were focused on the lesions present in these nuclei to see in which of the six stages the AD-related cytoskeletal anomalies begin and whether a correlation exists between the AD-related pathology developing within the cerebral cortex and the cytoskeletal damage that occurs in the nuclei of the raphe system. To this end, serial sections from the brainstems of 27 post-mortem cases with stages I-VI of cortical cytoskeletal lesions were examined. The cytoskeletal pathology was visualized using the modified silver iodide-Gallyas staining technique and the antibody AT8. The latter is directed specifically against the abnormally phosphorylated cytoskeletal protein tau. The dorsal raphe nucleus manifests the cytoskeletal lesions early on (stages I-II). The central and linear raphe nuclei, by contrast, do so initially in stages III-IV, and the caudal raphe nuclei register the first changes in stages V-VI. In stages V and VI, the dorsal raphe nucleus displays the most severe cytoskeletal pathology within the raphe system, followed by the central and linear raphe nuclei, whereas the cytoskeletal anomalies in the caudal raphe nuclei are slight. The developing damage within the nuclei of the raphe system correlates with the stages I-VI and, furthermore, progresses in the oral raphe nuclei in close connection with the evolution of the pathological process in cortical projection destinations of these nuclei. As the source of the ascending serotonergic system, the involvement of the oral raphe nuclei may be partially responsible for the early manifestation of the non-cognitive and emotional deficiencies possibly traceable to dysfunctions within the ascending serotonergic system.

Presence of apolipoprotein E immunoreactivity in degenerating neurones of mice is dependent on the severity of kainic acid-induced lesion

Apolipoprotein E (apoE) is a major apolipoprotein in the central nervous system (CNS) that may play a role in various CNS disorders. ApoE is primarily localised in astrocytes, but neuronal apoE mRNA expression has been demonstrated in normal and diseased human brain, as well as in ischaemic rat brain. To obtain further insight into the role of apoE in neuronal degeneration in the CNS and conditions of neuronal apoE localisation, we have investigated in mice the distribution of apoE following neuronal injury induced by kainic acid (n=35, 25 or 35 mg kainic acid/kg BW). Consecutive series of brain sections were immunostained for apoE and markers for astroglia (GFAP) and microglia/macrophage cells (CR3). Degenerating neurones were identified with a silver-degeneration staining technique. The intensity and cellular distribution of apoE-immunoreactivity (apoE-ir) was dependent on the severity of neuronal injury. Mice that developed mild neuronal degeneration, restricted to a subset of neurones in the hippocampus, showed increased apoE-ir in astrocytes concomitant with increased GFAP-ir and mild microgliosis. In these mice, no neuronal apoE-ir was detected. In contrast, mice developing severe neuronal injury in the hippocampus - frequently also showing degeneration in other brain regions including cortex, thalamus, striatum and amygdala - showed intense apoE-ir in degenerating neurones. Surrounding the lesion, apoE-ir was increased in neuropil recurrently whereas GFAP-ir astrocytes disappeared. Thus, in mice apoE accumulates in degenerating neurones in conditions of severe neuronal injury putatively in association with disruption of the glial network.

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

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

Quantitative distribution of parvalbumin, calretinin, and calbindin D-28k immunoreactive neurons in the visual cortex of normal and Alzheimer cases

The distribution of parvalbumin (PV), calretinin (CR), and calbindin (CB) immunoreactive neurons was studied with the help of an image analysis system (Vidas/Zeiss) in the primary visual area 17 and associative area 18 (Brodmann) of Alzheimer and control brains. In neither of these areas was there a significant difference between Alzheimer and control groups in the mean number of PV, CR, or CB immunoreactive neuronal profiles, counted in a cortical column going from pia to white matter. Significant differences in the mean densities (numbers per square millimeter of cortex) of PV, CR, and CB immunoreactive neuronal profiles were not observed either between groups or areas, but only between superficial, middle, and deep layers within areas 17 and 18. The optical density of the immunoreactive neuropil was also similar in Alzheimer and controls, correlating with the numerical density of immunoreactive profiles in superficial, middle, and deep layers. The frequency distribution of neuronal areas indicated significant differences between PV, CR, and CB immunoreactive neuronal profiles in both areas 17 and 18, with more large PV than CR and CB positive profiles. There were also significantly more small and less large PV and CR immunoreactive neuronal profiles in Alzheimer than in controls. Our data show that, although the brain pathology is moderate to severe, there is no prominent decrease of PV, CR and CB positive neurons in the visual cortex of Alzheimer brains, but only selective changes in neuronal perikarya.

The role of neuronal growth factors in neurodegenerative disorders of the human brain

Recent evidence suggests that neurotrophic factors that promote the survival or differentiation of developing neurons may also protect mature neurons from neuronal atrophy in the degenerating human brain. Furthermore, it has been proposed that the pathogenesis of human neurodegenerative disorders may be due to an alteration in neurotrophic factor and/or trk receptor levels. The use of neurotrophic factors as therapeutic agents is a novel approach aimed at restoring and maintaining neuronal function in the central nervous system (CNS). Research is currently being undertaken to determine potential mechanisms to deliver neurotrophic factors to selectively vulnerable regions of the CNS. However, while there is widespread interest in the use of neurotrophic factors to prevent and/or reduce the neuronal cell loss and atrophy observed in neurodegenerative disorders, little research has been performed examining the expression and functional role of these factors in the normal and diseased human brain. This review will discuss recent studies and examine the role members of the nerve growth factor family (NGF, BDNF and NT-3) and trk receptors as well as additional growth factors (GDNF, TGF-alpha and IGF-I) may play in neurodegenerative disorders of the human brain.

Brain-derived neurotrophic factor is reduced in Alzheimer's disease

Alzheimer's disease may be due to a deficiency in neurotrophin protein or receptor expression. Consistent with this hypothesis, a reduction in BDNF mRNA expression has been observed in human post-mortem Alzheimer's disease hippocampi. To further investigate this observation, we examined whether the alteration in BDNF expression also occurred at the protein level in human post-mortem Alzheimer's disease hippocampi and temporal cortices using immunohistochemical techniques. We observed a reduction in the intensity and number of BDNF-immunoreactive cell bodies within both the Alzheimer's disease hippocampus and temporal cortex when compared to normal tissue. These results support and extend previous findings that BDNF mRNA is reduced in the human Alzheimer's disease hippocampus and temporal cortex, and suggest that a loss of BDNF may contribute to the progressive atrophy of neurons in Alzheimer's disease.

Pathology of subcortical visual centres in relation to cortical degeneration in Alzheimer's disease

Subcortical visual centres such as the lateral geniculate nucleus, the lateral inferior pulvinar and the superior colliculus, together with the primary visual cortex and its adjacent white matter, were studied in 12 Alzheimer brains and five age-matched controls. The periodic acid methenamine technique was used for the demonstration of senile plaques and the Gallyas technique for neurofibrillary tangles and neuritic threads in the neuropil. In the lateral geniculate nucleus and inferior pulvinar, the presence of periodic acid methenamine-positive senile plaques was observed in variable numbers in all Alzheimer cases. In the lateral geniculate nucleus, senile plaques were encountered more often in parvocellular than in magnocellular layers, in the interlaminar zones, in the optic radiation and in the adjacent pre-geniculate nucleus. Gallyas staining did not reveal any neurofibrillary tangles, neuritic threads or neuritic plaques, meaning that in this thalamic region there are mainly amyloid deposits without neuritic degeneration. In the superior colliculus both amyloid and neuritic plaques, as well as neurofibrillary tangles and neuritic threads were encountered in the superficial and deep layers. In the primary visual cortex, all types of senile plaques were observed as well as a rather high number of neurofibrillary lesions in pyramidal neurons, mainly in layers 5 and 6, but also in several types of non-pyramidal neurons. In the underlying white matter there was a morphologically heterogeneous population of neurofibrillary tangle-bearing neurons and a considerable number of threads representing degenerating axons, suggesting that degeneration could follow corticosubcortical connections. These data demonstrate that lesions in the primary visual structures and pathways are more prevalent than previously observed and could partly explain the visual disturbances in Alzheimer's disease.

Attenuation of muscarinic receptor-G-protein interaction in Alzheimer disease

Cortical M1 muscarinic receptor-G-protein coupling, high-affinity, guanine nucleotide-sensitive agonist binding (Flynn et al., 1991; Warpman et al., 1993) and muscarinic receptor-stimulated [3H]PIP2 hydrolysis (Ferrari-DiLeo and Flynn, 1993) are known to be defective in Alzheimer disease. Whether this defect reflects an alteration in the M1 muscarinic receptor, its respective guanine nucleotide binding (G) protein or both is not known. This study compares the number and both basal and muscarinic receptor-mediated function of G-proteins in synaptosomal membranes from cerebral cortical samples of age-matched control subjects and Alzheimer disease patients. Immunoblotting with anti-G alpha q/11 and anti-G beta antibodies demonstrated no alteration in the number of these G-protein subunits in Alzheimer disease. Basal [35S]GTP gamma S binding and hydrolysis of [gamma-32P]GTP by high-affinity GTPase also were not significantly altered in Alzheimer disease compared to control membrane samples. However, muscarinic agonist-stimulated GTP gamma S binding and GTP hydrolysis were significantly reduced (80-100%) in Alzheimer disease cortical samples. Diminished agonist-stimulated GTP gamma S binding and GTP hydrolysis correlated with the loss of guanine nucleotide-sensitive, high-affinity agonist binding (KL/KH) ratio) to the M1 receptor subtype. These data provide further evidence for the loss of muscarinic receptor-G protein coupling in Alzheimer disease and support the hypothesis that muscarinic receptor-mediated cortical activation may be compromised in Alzheimer disease.

Acetylcholinesterase provides deeper insights into Alzheimer's disease

This article reviews the considerable evidence which rejects the cholinergic hypothesis of Alzheimer's disease (AD) and proposes that it is the AChE system of which the lightly stained neurons are located in the entorhinal cortex, the CA1/subiculum of the hippocampus and the amygdala which are the most vulnerable and are the earliest affected in the pathological processes of AD. Changes then spread out to the intermediately stained neurons of the association cortex, until they affect the heavily stained cells of the motor cortex. In general, senile plaque, a hallmark of AD, may be formed from the terminals of AChE-containing neurons. Neurofibrillary tangle, another hallmark of AD, may be formed in the perikarya of AChE-containing cells and bring about the demise of the neuron, thus leading to dementia.