Romanian Journal of Psychopharmacology (2010) 10, 1-8

PATHOPHYSIOLOGY OF ALZHEIMER’S DISEASE

Monica Gheorghita1, Iosif

Gabos-Grecu

1,2, Gabriela Buicu

1,2, Marieta

Gabos-Grecu

1,2,

Andreea Salcudean1, Ana Maria Todoran

2

1University Clinic of Psychiatry I Tirgu Mures, Romania

2University of Medicne and Pharmacy Tirgu Mures, Romania

Abstract Introduction. Alzheimer’s disease is a progressive dementia with loss of neurons and the presence of two main microscopic neuropathological hallmarks: extracellular amyloid plaques and intracellular neurofibrillary tangles. Propose. The present study proposes to present the major physiopathological mechanisms involved in the pathogenesis of the Alzheimer’s disease. Material and methods. Review of genetic, pathophysiological and neuropsichopharmacological changes, which occur during on evolution of this disease.The only confirmed risk factors for sporadic AD are age and the presence of the E4 allele of APOE (apolipoprotein E). Amyloid plaques comprise mainly of the neurotoxic peptide amyloid (A beta), cleaved sequentially from a larger precursor protein (APP) by two enzymes: Beta-secretase (also called BACE1) and Gama-secretase (comprising four proteins, one of which is presenilin). If APP is first cleaved by the enzyme alfa-secretase rather than beta-secretase then A- beta is not formed. Neurofibrillary tangles comprise mainly of the protein tau which binds microtubules, thereby facilitating the neuronal transport system. Uncoupling of tau from microtubules and aggregation into tangles inhibits transport and results in microtubule disassembly.Phosphorylation of tau may play an important role in this. Conclusion. Selective vulnerability of neuronal systems such as the cholinergic, serotonergic, noradrenergic and glutamatergic systems form the basis of current rational pharmacological treatment. Key words: Alzheimer’s disease, amyloid plaques, neurofibrillary tangles, apolipoprotein E, tau protein phosphorylation.

Introduction

Alzheimer’s disease is a progressive dementia with loss of neurons and the presence of two

main microscopic neuropathological hallmarks: extracellular amyloid plaques and intracellular

neurofibrillary tangles.

Propose

The present study proposes to present the major physiopathological mechanisms involved in

the pathogenesis of the Alzheimer’s disease.

1 Correspondence: Monica Gheorghita, University Clinic of Psychiatry I, Gheorghe Marinescu St. 38, Tg. Mures,

Romania. Tel +40 720-999764, e-mail: monica.gheorghita@hotmail.com

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Monica Gheorghita et al.

Material and methods

Review of genetic, pathophysiological and neuropsichopharmacological changes, which

occur during on evolution of this disease.

Neuropathology

The distribution and degree of severity of neuropathological changes seen post mortem in

brain tissue from AD patients is able to explain many of the symptoms seen in life. Generally there

is atrophy, with a reduction in gross size, particularly in the temporal lobe and hippocampus:

thinning of the cortical gyri and enlargement of the third and lateral cerebral ventricles.

Microscopic analysis reveals the characteristic extracellular amyloid plaques and

intracellular neurofibrillary tangles. At least 80% of cases also show congophilic angiopathy, the

deposition of cerebrovascular amyloid in the small vessels in the leptomeninges and cortices. The

peptide A-beta is the major constituent of the amyloid plaque. In some plaques it is in a beta-pleated

sheet form which can be visualized by stains such as Congo red or the fluorescent dye Thioflavin S.

Neuronal loss and/or pathology may be seen particularly in the hippocampus, amygdale,

entorhinal cortex and the cortical association areas of the frontal, temporal and parietal cortices, but also

in subcortical nuclei such as the serotonergic dorsal raphe, noradrenergic locus coeruleus, and the

cholinergic basal nucleus. The motor area, primary visual areas, and sensory regions are largely spared.

The deposition of tangles follows a defined pattern, beginning in the trans-entorhinal cortex;

subsequently the entorhinal cortex, the CA1 region of the hippocampus and later the cortical

association areas, where frontal, parietal and temporal lobes are particularly affected. There is

evidence that neurofibrillary tangle formation occurs subsequent to A-beta accumulation and since

plaques are found at the terminals of neurons with intracellular tangles it is likely that amyloid at

the dendritic/axonal synapses has an effect retrogradely, such that tangles subsequently form within

those neurons. The extent and placement of tangle formation correlates well with the severity of

dementia, much more so than numbers of amyloid plaques. However the reason may be that it is

easier to clear extracellular plaques as there are well defined clearing mechanisms such as the action

of the enzymes insulin-degrading enzyme (IDE), angiotensin-converting enzyme (ACE), neutral

endopeptidase (neprilysin), and tissue type plasminogen activator (tPA). In fact accumulation of

amyloid in some cases of sporadic AD may be related to a deficit in this amyloid clearance system.

Neurochemical changes

Much of the cholinergic innervation of the cortex and hippocampus originates from a

nucleus (approximately 2 cm long) in the basal forebrain: this is the nucleus basalis of Meynert

(nbM). There is a loss of cholinergic function very early in the AD process, which results in an

early loss of short term memory function.

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Pathophysiology of Alzheimer’s Disease

Cholinesterase inhibitors, the main current therapeutic for AD, potentiate the cholinergic

signal by preventing degradation of the neurotransmitter acetylcholine by the enzyme

acetylcholinesterase.

The cholinergic cells of the nbM are responsive to a protein called nerve growth factor

(NGF); withdrawal of this results in a neuronal atrophy. There are two types of receptor

responsive to NGF: p75NTR and tyrosine receptor kinase A (TrkA). Early in the disease

process the number of TrkA positively staining cells is reduced. Recently it has been shown that

NGF in human brain is in the pro form, i.e.pro-NGF, and this is increased in AD. Evidence

suggests that pro-NGF binding to p75NTR, in conjunction with binding to another receptor,

sortilin, is associated with apoptosis. However, levels of a related protein brain-derived

neurotrophic factor (BDNF) are reduced in AD brain; along with their receptor TrkB. BDNF is

important in synaptic plasticity, memory and learning. Both NGF and BDNF-related

therapeutics may therefore form a basis for future therapeutics.

Glutamatergic neurons of the hippocampus and cortex are also affected and may also result

in memory disfunction; and loss of noradrenergic and serotonergic neurons in the locus coeruleus

and dorsal raphe respectively, may be associated with bahavioural changes such as depression.

Familial Alzheimer’s disease: genetic clues

Amyloid precursor protein: chromosome 21. Amyloid extracted from either cerebrovascular

amyloid or parenchymal plaques in AD brain is a 4kDa peptide derived from a much larger amyloid

precursor protein (APP). The APP gene is localized on chromosome 21, perhaps explaining why

Down’s syndrome (Trisomy 21) sufferers usually develop AD symptoms and pathology by their

forties, since they get an extra copy of the gene.

Over 30 mutations in APP are currently listed, not all of which are pathogenic. Some

however do lead to autosomal dominant forms of AD (familial/early onset AD (FAD)) or to

cerebrovascular disease. The peptide A-beta is cleaved from APP by two enzymes beta-secretase

(identified now as BACE1 or beta-site APP cleaving enzyme1 ) and gamma-secretase (comprising

four proteins, one of which is presenilin). This forms the so called “amyloidogenic pathway. If,

however, another enzyme, alfa-secretase, cleaves APP within the A-beta peptide (between

K16/L17), it prevents the formation of A-beta. This is the “non-amyloidogenic pathway”.

FAD mutation in APP result in an increase or alteration in A-beta produced. For instance,

the “ Swedish “ double mutation (KM/NL) at the N-terminal of the A-beta cutting site results in an

increase in total A-beta production. This is due to the increased affinity of BACE1 for the altered

amino acid sequence. Mutations at or near the C-terminal of the A-beta peptide sequence (e.g.

V717I; the “London “ mutation), near the gamma-secretase cleavage site result in a change in the

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type of A-beta produced; so a 42 amino acid variant of A-beta (A-beta1-42) is favoured over the

more common forty amino acid variant (A-beta 1-40).

A-beta 1-42 has been shown to be a more neurotoxic form and is more easily able to

aggregate. It is usually found within plaques in the parenchyma rather in the cerebrovasculature,

probably due to its propensity to rapidly aggregate. By contrast A-neta 1-40 is sufficiently soluble

to be cleared to the blood vessels before depositing and so comprises the majority of

cerebrovascular amyloid. Mutation in the middle of the A-beta sequence result in either pure

cerebrovascular pathology without plaques or tangles such as in hereditary cerebral haemorrhage

with amyloidosis-Dutch type (HCHWA-D), or cerebrovascular/AD mixed pathology as seen in

“Flemish” dementia.

Presenilin mutations: chromosomes 1 and 14

Genetic linkages have been found in early onset AD kindred with the gene PS1 (also called

PSEN1) on chromosome 14 and PS2 (PSEN2) on chromosome 1. These genes code for the proteins

presenilin 1 (PS1) and presenilin 2 (PS2), respectively, which are 66% homologous. Mutations in

PS1 and PS2, like APP mutations, are autosomal dominant, although in general PS1 mutations

cause more severe forms with an earlier age of onset. Some mutations in PS1 have been shown to

cause symptoms in the 20s and 30s. There are approximately 180 PS! Mutations recorded, most of

which are pathogenic. These mutations have been shown to be distributed throughout the gene, yet

all are thought to result in an increase in the A-beta 1-42; A-beta 1-40 ratio. By contrast there are

only just over 20 mutations discovered in the PS2 gene, not all are pathogenic, and symptoms do

not generally appear until the forties or fifties.

Although familial early onset AD accounts only for about 1% of all cases of AD the

underlying mechanisms may provide clues as to how the pathology may develop in sporadic AD.

The processing of APP plays a key role in the underlying pathology both in familial forms and

sporadic forms of the disease. An increase in gene dosage, such as in Down’s syndrome, or an

increase in total A-beta peptide produced, or an increase in the A-beta 1-42 form of the peptide

result in AD pathology and symptoms. There is also evidence that in many sporadic forms of AD

there is an increase in activity of BACE1 and this may account for the accumulation of amyloid

plaques. The importance of A-beta formation is highlighted in the “amyloid cascade hypotesis”

which argues for the initial formation of A-beta setting off a cascade of events including formation

of neurofibrillary tangles resulting symptoms of dementia.

Amyloid toxicity

Amyloid formation occurs with the sequential formation of dimmers, oligomers, and finally

polymers in a concentration-dependent manner, the initial slow phase of nucleation may be speeded

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Pathophysiology of Alzheimer’s Disease

up by “seeding” due to catalysts such as apolipoprotein E, ions (such as Fe 3+) or

glycosaminoglycans which probably help to induce a beta-strand formation. Aggregation of

amyloid may cause an inflammatory response which contributes to the pathology, since activated

microglia and astrocytes associate with neuritic plaques, and there is an increase in inflammatory

mediators such as C1q in the complement cascade, and cytokines such as TNF-alfa, interleukin 1-

beta (IL 1-beta), transforming growth factor beta-1 (TGF-beta-1) and IL6.

However A-beta oligomers are also known to be toxic to cells, partly due to oxidative stress,

disruption of calcium homeostasis and oxidative damage to DNA, lipids and proteins.A receptor has

been identified which may mediate the toxic effects of A-beta and cause intracellular damage.

RAGE (receptor for advanced glycation endproducts) is a cell surface receptor present in AD brain

in abnormally high levels in neurons, microglia, and astrocytes, particularly close to amyloid

plaques and in tangle-bearing neurons. A-beta binds to the RAGE receptor, causing nuclear

translocation of NFk-beta and increased expression of cytokines, such as TNF-alfa and macrophage

colony-stimulating factor (M-CSF).

The presence of A-beta increases oxidative stress with generation of intracellular superoxide

radicals and H2O2. Cumulative damage results including oxidation of lipids and proteins. Thus A-

beta has toxic effects by inducing inflammatory response and intracellular damage. It also probably

interferes with neurotransmission by binding at synapses and also by specifically binding to and

with important neuronal proteins.

Tau: hyperphosphorylation and pathogenicity

Neurofibrillary tangles comprise largely of paired helical filaments (PHF), the major

component of which is a 12kDa fragment of the protein tau. Tau is a microtubule associated protein

which helps to support the microtubule network in neurons by facilitating assembly and

stabilization of microtubules. It exists in six isoforms (352-441 amino acids); three isoforms contain

three tandem microtubule-binding domain repeat regions and three isoforms have four repeat

regions. The gene for tau (MAPT) is located on chromosome 17.

Although tau plays a central role in AD, and numbers of tangles correlate significantly with

degree of dementia, mutations in tau do not lead to AD. Instead they produce dementias such as

fronto-temporal dementia with Parkinsonism-17, which is associated with chromosome 17 (FDTP-

17). Pick’s disease sufferers include those with FDTP-17. The fact that amyloid plaques are not

necessary for cell death and dementia implies that although A-beta production is probably primary

in the sequence of pathogenic events, the production of neurofibrillary tangles may be more

important in cell death and the symptoms of dementia. In fact most models of AD show virtually no

tau-associated pathology, and little neuronal death. Tau is both hyperphosphorylated and

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abnormally phosphorylated in AD brain and there is evidence to suggest that this abnormal

hyperphosphorylation of tau leads to its loss pf normal function i.e. binding to microtubules is

inhibited which reduces its ability to promote microtubule assembly. Recently triple transgenic

models have been produced with mutations in human APP, PS1, and MAPT genes. These have

accelerated signs of AD pathology including hyperphosphorylated tau and tangles.

A number of kinases, including praline-directed kinases (PDPKs), may be involved in

phosphorylation of tau in vivo, including glycogen synthase kinase-3 beta (GSK-3Beta), and cyclin-

dependent protein kinase-5 (cdk5) which are associated with AD neurofibrillary pathology. The

balance between phosphorylation by kinase activity and dephosphorylation by phosphatase activity

is crucial. There is a reduction in phosphatase activity in AD brain; in particular the phosphatase

PP-2A is reduced by 20-30% in AD brain. Additionally, Pin 1 a peptidyl-prolyl cis/trans isomerase,

is able to regulate the function of tau and also the processing of APP. Pin 1 is reduced in AD brain

and the knockout mouse shows tau and A-beta-related pathology and it has been suggested that this

provides a link between A-beta and formation of tangles.

Apolipoprotein E: chromosome 19

Amyloid plaques and neurobibrillary tangles both stain positive by

immunohistochemistry with an antibody to the protein apolipoprotein E. By linkage studies

late-onset AD has been shown to be associated with the presence of APOE4, a polymorphism of

the APOE gene, located on chromosome 19. Apolipoprotein E is a glycoprotein which

transports cholesterol and phospholipids as high density lipoprotein (HDL) particles and

mediates uptake into cells via the low density lipoprotein receptor (LDLR) and the LDL-

receptor-related protein (LRP). There are three common isoforms of apolipoprotein E: E2, E4,

and most frequent in the population is E3. The protein for which they each code differs such

that apolipoprotein E3 has the amino acid cysteine at position 112 and arginine at 158, E2 has

cysteine residues at both positions, whereas E4 has arginine residues.

The presence of the E4 allele is a risk factor for sporadic AD; statistically there is an

increased frequency of the E4 allele in sporadic AD compared with age-matched normal subjects

(approximately 40% versus 16% respectively). The presence of one E4 allele reduces the theoretical

age of onset of the disease by 5 years, the presence of two E4 alleles statistically will reduce the age

of onset by 10 years. The E2 allele, by contrast, has been suggested as having a protective effect.

The presence of E4 has been likened to a “knockout” of apolipoprotein E; it is associated

with lower synaptic density, even in normal adult brain, and also with a reduced repair response

after injury and higher mortality rate. Whereas E2 and E3 preferentially bind to the smaller

phospholipids rich in HDL, because of the conformation of E4 it preferentially binds larger

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Pathophysiology of Alzheimer’s Disease

triglyceride-rich very-low density lipoproteins (VLDL). Carriers of E4 therefore tend to have higher

cholesterol levels. This may result in vascular compromise; it may also result in a reduced synapse

formation. Lipids are required for reparation and remodeling of synapses throughout life and

especially after injury; if E4 is less able to perform the function of lipid transport this may lead to

impaired synapse formation and effectively reduce the so called “neuronal reserve” wich maintains

us above the threshold of cognitive impairment.

Additionally, all forms of apolipoprotein E assist aggregation of AB by facilitating its

conversion from alfa-helix to beta-strand conformation. Evidence suggests that lapidated

apolipoprotein E keeps Aß soluble, but when it is poorly lapidated, and this is particularly true of

E4, it facilitates an increase in Aß fibril formation.

Furthermore, apolipopreotein E is able to bind to neurofibrillary tangles as well as to

amyloid. E3 forms more stable complexes with tau in vitro than E4 does and this may protect tau

from hyperphosphorylation.

Thus the presence of the E4 isoform of apolipoprotein E has potentially serious long term

effects for neuronal reserve, recovery from neuronal injury, increased Aß fibrilization and tau

phosphorylation. Which of these is most important is not yet clear, maybe all aspects contribute to

its risk in AD.

Other risk factors for sporadic Alzheimer’s disease

Increased age and the presence of the E4 isoform of apoE are the only confirmed risk factors

for sporadic AD. However a number of other genetic predispositions and environmental issues

(including diet) have been investigated. AD has been called a brain-specific “Type 3 diabetes”, and

there are an increasing number of links berween diabetes and AD including dysregulation of insulin

signaling in the brain. Genetic association studies have implicated several genes. Some of these are

involved with production or clearance of Aß; others such as the nicotinic cholinergic receptor,

cholesterol metabolism and tau have understandable links; however the reason for a link with many

of the other genes is still unclear.

Conclusion

The symptoms and the neuropathology of AD form a spectrum, with the likehood of diverse

underlying mechanisms. However the formation and clearance of Aß are important as this toxic

peptide leads to neuronal damage and eventually the formation of neurofibrillary tangles. Success in

future therapeutic avenues will benefit from an understanding of how the many risk factors are able

to affect the molecular pathology to produce this progressive dementia.

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