CHAPTER NINE
DEGENERATIVE DISEASES

ALZHEIMER'S DISEASE

Alzheimer's Disease is a specific neurodegenerative disease and is the most common cause of dementia in old people. Clinically, it is characterized by loss of memory, inability to learn new things, loss of language function, a deranged perception of space, inability to do calculations, indifference, depression, delusions, and other manifestations. It is inexorably progressive and fatal within 5 to 10 years. AD patients usually die of complications of chronic illness. AD is the fourth to fifth most common cause of death in the United States. Sometimes AD involves people in their 40s and 50s, but is mainly a disease of old age. Its overall incidence in persons over 65 is approximately 10%. Its incidence in persons over 85 approaches 50%.

PATHOLOGY OF AD

AD is characterized by deposition of abnormal, insoluble extracellular (beta amyloid-Aß) and intracellular (tau) proteins. Aß is the main component of senile plaques and tau is the component of neurofibrillary tangles. Aß deposition is specific for AD. Neurofibrillary change is also seen in other degenerative diseases.

Aß is a 40 to 42 amino acid peptide, which is part of a larger protein, the Amyloid Precursor Protein (APP). APP is a transmembrane protein, made by neurons and other brain cells. It is also found in extraneural tissues and is especially abundant in platelets. Its function is unknown. The Aß amyloid residue includes part of the transmembrane domain of APP and is derived by the successive cleavage of APP by the enzymes beta-secretase and gamma-secretase. Accumulation of Aß is thought to result from aberrant cleavage of APP by these secretases and other mechanisms (see genetics, further on). Altered structure or increased amounts cause Aß peptides to aggregate, change their folding pattern, and precipitate as amyloid fibrils.

Aß is toxic to neurons. In brain slice preparations, it causes loss of long term potentiation, damages synapses, and kills neurons. Moreover, it shows selective neurotoxicity for the hippocampus and entorrhinal cortex (areas that are severely affected in AD) while sparing cerebellar neurons. This damage is mediated by free radicals, which are generated when soluble Aß is complexed with Zn2+, Cu2+, and Fe3+. There is a high correlation between the amount of soluble Aß and the severity of the neurological dysfunction in AD. In transgenic AD models, severe neurological deficits occur in absence of amyloid deposits in tissue.

Neurofibrillary degeneration is characterized by the deposition in the neuronal body and processes of insoluble polymers of over-phosphorylated microtubule associated protein tau. Tau aggregates as pairs of filaments that are twisted around one another (paired helical filaments). These deposits interfere with cellular functions by displacing organelles. By destabilizing the cytoskeleton, they impair the axoplasmic flow and affect the nutrition of axon terminals and dendrites.

Alzheimer plaques-Bielschowski stain	Alzheimer plaques. Beta amyloid immunostain

There are two main lesions in AD, senile plaques (SPs) (also called Alzheimer's plaques) and neurofibrillary tangles (NFTs). SPs are spherical lesions in the cerebral cortex, measuring up to 100 microns. In their fully developed stage-the neuritic plaque- SPs have a central core of extracellular amyloid surrounded by dystrophic neuronal processes with neurofibrillary degeneration. The zone around the amyloid core contains also reactive astrocytes and microglia. The amyloid core in these plaques has a fibrillary fine structure and is Congo Red positive and birefringent, similar to other amyloids. In addition to neuritic plaques, Aß is also found in diffuse, non-fibrillar deposits (diffuse plaques). Diffuse plaques do not disrupt the neuropil. They are seen sometimes in large numbers in old, non-demented persons and are not associated with dementia. Many AD patients have also cerebral amyloid angiopathy (see further on).

Neurofibrillary tangles-Bielschowski stain	Paired helical filaments

NFTs are deposits of tau filaments in the neuronal body. Similar deposits are present in the dystrophic processes that surround the amyloid core of SPs and in dendrites (neuropil threads). In severe AD, the hippocampus often contains extracellular NFTs embedded in the neuropil, like fossilized skeletons of neurons. The mechanism of accumulation of tau in NFTs is unclear.

The prevailing opinion about the pathogenesis of AD is that the primary lesion is Aß deposition and NFTs are secondary. The finding of NFTs in diverse other neurodegenerative diseases (see Frontotemporal Dementias and Tauopathies) lends support to this view. However, mutations of the tau gene have been described, which cause neurofibrillary pathology with dementia and parkinsonism. These cases indicate that NFTs can cause neurodegeneration indepenently of Aß deposition.

Most cases of AD show a combination of SPs and NFTs, but some cases have a predominance of one or the other. Neurofibrillary pathology first develops in the entorhinal cortex and, as the disease progresses, it spreads to the hippocampus and amygdala and then to the neocortex.

Severe AD	Cortical atrophy	Hydrocephalus ex vacuo

Amyloid deposits are more frequent in association cortex. In very severe cases, SPs and NFTs appear also in deep nuclei and in the brain stem. There are no such lesions in the white matter.

Each SP represents a focus of damage of the neuropil that includes axon terminals and dendrites of several neurons and probably thousands of synapses. Thus, SPs cause severe disconnection. The distribution of the lesions correlates with the clinical picture. Damage of the hippocampus explains the impairment of memory, and involvement of association cortex correlates with the loss of higher intellectual functions. SPs and NFTs are associated with loss of neurons and synapses, brain atrophy, and dilatation of the lateral ventricles due to loss of brain tissue (hydrocephalus ex vacuo).

The lesions of AD can be best appreciated in sections of brain stained with the Bielschowski silver stain, the fluorescent stain thioflavin S, and immunostains for Aß. NFTs and neuropil threads fluoresce and stain black with the Bielschowski stain. Aß immunostains highlight the amyloid cores of SPs and reveal also diffuse amyloid deposits without disruption of the neuropil (diffuse plaques). Many AD cases show also vascular amyloid deposits.

GENETICS OF AD

The gene for APP is on chromosome 21. Trisomy 21 (Down syndrome) provides a clear mechanism for Aß deposition. Persons with this condition produce one and a half times as much APP as normal people do and develop AD at a young age, some of them in their 20s. Most AD, however, is not due to overproduction of APP, but to some other mechanism, either an abnormality of the APP molecule that renders it more amyloidogenic, or a defect of processing of normal APP. This appears to be the case in infrequent genetic forms of AD. In these patients, autosomal dominant AD develops before age 65 (presenile dementia) due to mutations of the APP gene and of the presenillin 1 and 2 genes on chromosomes 14 and 1 respectively. The presenillins are thought to affect secretase activity.

Setting aside Down syndrome and autosomal dominant AD, the vast majority (99%) of AD cases appear in old age and are probably due to an interaction of genetic and other intrinsic and environmental factors. The most important genetic risk factor is the Apolipoprotein E (ApoE) genotype. ApoE is a protein that carries lipids in and out of cells. It occurs in three isoforms: ApoE2, ApoE3, and ApoE4. The gene for ApoE is on chromosome 19. One copy is inherited from each parent. The most common ApoE allele is ApoE3. Persons who are homozygous for the ApoE4 allele develop AD at a mean age 70. Persons with other ApoE phenotypes develop the disease later. The ApoE4 allele is also a risk factor for hypercholesterolemia. High cholesterol levels during mid-life increase the risk of AD and lipid-lowering drugs lower this risk. ApoE4 has been detected in NFTs and in Aß. These findings suggest that ApoE lipoproteins participate in some way in the processing of APP, perhaps by modulating APP secretases, and may also play a role in the assembly of the neuronal cytoskeleton.

RISK FACTORS

The role of the environment, diet, and general state of health in AD is now beginning to be explored. Chronic cellular damage from free radicals, excitotoxicity, nonenzymatic glycation of proteins, and other factors contributes to the loss of neurons and synapses that is associated with old age and aggravates the pathology of AD. Increased levels of homocysteine (also a risk factor for stroke) and decreased dietary folate potentiate these neurotoxic effects. Homocysteine increases with advancing age and is elevated in persons with polymorphisms of 5,10-methylenetetrahydrofolate reductase (MTHFR), an important enzyme involved in folate metabolism. Such polymorphisms are very common. Elevated homocysteine and decreased folate are associated with increased free radicals, cytosolic calcium, glutamate excitotoxicity, apoptosis, and decreased levels of ATP.

There is also evidence that inflammatory and immune mechanisms are involved in the pathogenesis of AD. Acute-phase proteins are elevated in serum and deposited in SPs; microglial cells accumulate around SPs; and complement components are present in SPs. APP is an acute phase protein which is released in brain tissue following trauma and other insults. Dementia and parkinsonism develop sometimes in boxers (dementia pugilistica or the punch drunk syndrome) and accelerated cognitive decline occurs with advancing age in persons with a history of traumatic brain injury. The brain in dementia pugilistica shows NFTs and diffuse plaques but not the typical neuritic plaques of AD.

The risk for developing AD in old age can be assessed partially based on the ApoE genotype. The contribution of general health and the environment is difficult to factor in, and there may be other genetic risk factors that we don't know about. Possibly, whether a person with ApoE4/4 develops AD at 70 vs 75 years depends on general health and the environment. Stroke, CNS infections, traumatic brain lesions, multiple sclerosis, and any type of brain damage deplete structural and functional reserves and aggravate the dementia.

Old age (without clinical dementia) is associated with some loss of neurons and synapses and an overall reduction of brain weight by 200 gm. The remaining neurons are enough to carry out neurological function. Some compensatory dendritic sprouting is also seen. Neuronal plasticity (the ability to make new synapses) is enhanced by trophic factors (neurotrophins). The best-known neurotrophin, nerve growth factor, is important for growth and maintenance of cholinergic neurons that are depleted in AD. Neuronal activity also enhances plasticity.

NEUROTRANSMITTERS IN AD

The brain, in AD, shows a loss of cholinergic neurons in the basal forebrain, decreased acetylcholine (Ach) levels, and a decrease in the acetylcholine synthesizing enzyme choline acetyltransferase (CHAT) in the cerebral cortex. Animal models show that Ach plays a crucial role in information processing and memory. Although other neurotransmitter systems (noradrenalin, serotonin, somatostatin and other peptides) are also deficient in AD, the cognitive impairment correlates best with the loss of cholinergic input. Acetylcholinesterase inhibitors (tacrine) and Ach receptor agonists, including nicotine, have been used to treat AD. The marginal success of this approach suggests that, in addition to Ach deficiency, there are other profound alterations that contribute to the cognitive dysfunction.

CEREBRAL PERFUSION IN AD

An important cause of neuronal dysfunction in AD is impairment of cerebral perfusion. Soluble Aß is a potent vasoconstrictor of cerebral vessels. Amyloid deposition on the vascular wall results in loss of smooth muscle and narrowing of the lumen. With the loss of their smooth muscle, cerebral arterioles lose their ability to constrict and dilate in response to regional brain activation. Narrowing of capillaries decreases cerebral perfusion and impairs the blood brain barrier function. Occluded capillaries form the seed of SPs. Thus, the brain capillary network is diminished and does not regenerate because of senescence. These pervasive changes are superimposed on cerebral atherosclerosis.

DIAGNOSIS OF AD

There are no specific clinical findings in AD. However, progressive dementia evolving over a few years without focal neurologic deficits or abnormal imaging findings is probably AD. Elevation of tau protein and decrease of Aß in the CSF are useful biomarkers. A definitive diagnosis can only be made by pathological examination of brain tissue. One of the most frequent reasons for requesting an autopsy today is in order to establish the diagnosis of AD. Autopsy studies show that the brains of most people over 65, even without clinical dementia, contain a few SPs and NFTs in the hippocampus and entorrhinal cortex. This suggests that formation of SPs and NFTs is part of the aging process. The brains of demented people contain more SPs and NFTs, not only in the limbic cortex but also in the neocortex and other regions. The more numerous and widespread the SPs and NFTs, the more severe the dementia.

Fibrillar Aß deposits can be imaged in vivo by PET scan, following administration of carbon 11 (11C)-labeled Pittsburgh Compound B (PiB), a substance that crosses the blood-brain barrier and binds selectively to Aß. PiB binding detects Aß in plaques and in blood vessels and gives an overall estimate of Aß load, including low-grade deposits in people who are not demented.

The finding of AD pathology in nondemented old people presents the neuropathologist with a dilemma. Should cases with a few SPs and NFTs be diagnosed as AD, even if there is no clinical dementia? The NIA-Reagan and CERAD diagnostic criteria for AD (see references below) take into account not only the numbers of SPs and NFTs but also age and intellectual function. The Braak and Braak system stages AD based on the extent of neurofibrillary pathology throughout the cortex. Certainly the presence of SPs and NFTs indicates that the biochemical abnormalities that cause AD are set in motion. In some individuals, this process advances rapidly and causes severe dementia; in others, it is slow and causes forgetfulness and a mild decline in mental power. The term mild cognitive impairment indicates a clinical state between normal ageing and dementia. MCI is clinically and pathologically diverse. However, many patients with MCI have mild AD changes and progress to full blown AD. While old age is the most important risk factor for AD, it is worth emphasizing that 90% of people over 65 have no clinical dementia. Despite significant advances in the past 20 years, major gaps in our knowledge of AD remain. Knowledge about the neurotransmitter deficiencies, neuronal plasticity, and the role of the environment and body-brain interactions may provide a basis for designing treatment protocols for AD.

A novel prevention and treatment method for AD was reported recently in transgenic mice that overexpress a mutant APP and develop AD neuropathology. Active immunization of young animals with Aß and passive immunization with Aß antibodies prevented the development of AD; immunization of older animals reduced the extent and severity of AD pathology. Based on these findings, a human vaccine consisting of synthetic Aß was developed. Phase I trials went uneventfully but phase II studies were terminated because some patients developed an autoimmune meningoencepalitis.

Further reading:
Consensus recommendations for the postmortem diagnosis of Alzheimer's disease. The National Institute on Aging, and Reagan Institute Working Group on Diagnostic Criteria for the Neuropathological Assessment of Alzheimer's Disease. Neurobiol Aging. 1997;18(4 Suppl):S1-2. PubMed

Mirra SS, Heyman A, McKeel D, et al. The Consortium to Establish a Registry for Alzheimer's Disease (CERAD). Part II. Standardization of the neuropathologic assessment of Alzheimer's disease. Neurology. 1991;41:479-86. PubMed

Braak H, Braak E. Neuropathological stageing of Alzheimer-related changes. Acta Neuropathol (Berl). 1991;1991;82:239-59. PubMed

Goedert M, Spillantini MG. A century of Alzheimer's disease. Science 2006;314:777-81. PubMed

Bacscai BJ, Frosch MP, Freeman SH, et al. Molecular imaging with Pittsburgh Compound B confirmed at autopsy. A case report. Arch Neurol 2007;64:431-34. PubMed

Castellani RJ, Lee HG, Zhu XJ, et al. Alzheimer Disease Pathology As a Host Response. J Neuropathol Exp Neurol. 2008;67:523-31. PubMed

Updated: June, 2008