Biochemistry of Alzheimer's disease

The biochemistry of Alzheimer's disease (AD), one of the most common causes of adult dementia, is not yet very well understood. AD has been identified as a possible protein misfolding disease due to the accumulation of abnormally folded amyloid beta protein in the brains of Alzheimer's patients.[1] Amyloid beta, also written Aβ, is a short peptide that is an abnormal proteolytic byproduct of the transmembrane protein amyloid precursor protein (APP), whose function is unclear but thought to be involved in neuronal development.[2] The presenilins are components of proteolytic complex involved in APP processing and degradation.[3]

Amyloid beta monomers are soluble and contain short regions of beta sheet and polyproline II helix secondary structures in solution,[4] though they are largely alpha helical in membranes;[5] however, at sufficiently high concentration, they undergo a dramatic conformational change to form a beta sheet-rich tertiary structure that aggregates to form amyloid fibrils.[6] These fibrils deposit outside neurons in dense formations known as senile plaques or neuritic plaques, in less dense aggregates as diffuse plaques, and sometimes in the walls of small blood vessels in the brain in a process called amyloid angiopathy or congophilic angiopathy.

AD is also considered a tauopathy due to abnormal aggregation of the tau protein, a microtubule-associated protein expressed in neurons that normally acts to stabilize microtubules in the cell cytoskeleton. Like most microtubule-associated proteins, tau is normally regulated by phosphorylation; however, in AD patients, hyperphosphorylated tau accumulates as paired helical filaments[7] that in turn aggregate into masses inside nerve cell bodies known as neurofibrillary tangles and as dystrophic neurites associated with amyloid plaques. Although little is known about the process of filament assembly, it has recently been shown that a depletion of a prolyl isomerase protein in the parvulin family accelerates the accumulation of abnormal tau.[8][9]

Neuroinflammation is also involved in the complex cascade leading to AD pathology and symptoms. Considerable pathological and clinical evidence documents immunological changes associated with AD, including increased pro-inflammatory cytokine concentrations in the blood and cerebrospinal fluid.[10][11] Whether these changes may be a cause or consequence of AD remains to be fully understood, but inflammation within the brain, including increased reactivity of the resident microglia towards amyloid deposits, has been implicated in the pathogenesis and progression of AD.

Neuropathology

At a macroscopic level, AD is characterized by loss of neurons and synapses in the cerebral cortex and certain subcortical regions. This results in gross atrophy of the affected regions, including degeneration in the temporal lobe and parietal lobe, and parts of the frontal cortex and cingulate gyrus.[12]

Both amyloid plaques and neurofibrillary tangles are clearly visible by microscopy in AD brains.[13] Plaques are dense, mostly insoluble deposits of protein and cellular material outside and around neurons. Tangles are insoluble twisted fibers that build up inside the nerve cell. Though many older people develop some plaques and tangles, the brains of AD patients have them to a much greater extent and in different brain locations.[14]

Biochemical characteristics

Fundamental to the understanding of Alzheimer's disease is the biochemical events that leads to accumulation of the amyloid-beta and tau-protein. A delicate balance of the enzymes secretases regulate the amyloid-beta accumulation. Alpha secretase activity discourages its accumulation. Recently, a link between cholinergic neuronal activity and the activity of alpha-secretase has been highlighted,[15] which can discourage Amyloid-beta proteins deposition in brain of patients with Alzheimer's Disease. Alzheimer's disease has been identified as a protein misfolding disease, or proteopathy, due to the accumulation of abnormally folded Amyloid-beta proteins in the brains of AD patients.[1] Abnormal amyloid-beta accumulation can first be detected using cerebrospinal fluid analysis and later using positron emission tomography (PET).[16]

Although AD shares pathophysiological mechanisms with prion diseases, it is not transmissible like prion diseases.[17] Amyloid-beta, also written Aβ, is a short peptide that is a proteolytic byproduct of the transmembrane protein amyloid precursor protein (APP), whose function is unclear but thought to be involved in neuronal development. The presenilins are components of a proteolytic complex involved in APP processing and degradation.[3] Although amyloid beta monomers are harmless, they undergo a dramatic conformational change at sufficiently high concentration to form a beta sheet-rich tertiary structure that aggregates to form amyloid fibrils[6] that deposit outside neurons in dense formations known as senile plaques or neuritic plaques, in less dense aggregates as diffuse plaques, and sometimes in the walls of small blood vessels in the brain in a process called amyloid angiopathy or congophilic angiopathy.

AD is also considered a tauopathy due to abnormal aggregation of the tau protein, a microtubule-associated protein expressed in neurons that normally acts to stabilize microtubules in the cell cytoskeleton. Like most microtubule-associated proteins, tau is normally regulated by phosphorylation; however, in AD patients, hyperphosphorylated tau accumulates as paired helical filaments[7] that in turn aggregate into masses inside nerve cell bodies known as neurofibrillary tangles and as dystrophic neurites associated with amyloid plaques.

Levels of the neurotransmitter acetylcholine (ACh) are reduced. Levels of other neurotransmitters serotonin, norepinephrine, and somatostatin are also often reduced. Replenishing the ACh by anti-cholinesterases is an approved mode of treatment by FDA. An alternative method of stimulating ACh receptors of M1-M3 types by synthetic agonists that have a slower rate of dissociation from the receptor has been proposed as next generation cholinomimetic in Alzheimer's disease[15].

Disease mechanism

Although the gross histological features of AD in the brain are well characterized, three major hypotheses have been advanced regarding the primary cause. The oldest hypothesis suggests that deficiency in cholinergic signaling initiates the progression of the disease. Two alternative misfolding hypotheses instead suggest that either tau protein or amyloid beta initiates the cascade. While researchers have not identified a clear causative pathway originating from any of the three molecular hypotheses to explain the gross anatomical changes observed in advanced AD, variants of the amyloid beta hypothesis of molecular initiation have become dominant among the three possibilities.

Cholinergic hypothesis

The oldest hypothesis is the "cholinergic hypothesis". It states that Alzheimer's begins as a deficiency in the production of acetylcholine, a vital neurotransmitter. Much early therapeutic research was based on this hypothesis, including restoration of the "cholinergic nuclei". The possibility of cell-replacement therapy was investigated on the basis of this hypothesis. All of the first-generation anti-Alzheimer's medications are based on this hypothesis and work to preserve acetylcholine by inhibiting acetylcholinesterases (enzymes that break down acetylcholine). These medications, though sometimes beneficial, have not led to a cure. In all cases, they have served to only treat symptoms of the disease and have neither halted nor reversed it. These results and other research have led to the conclusion that acetylcholine deficiencies may not be directly causal, but are a result of widespread brain tissue damage, damage so widespread that cell-replacement therapies are likely to be impractical.

More recent hypotheses center on the effects of the misfolded and aggregated proteins, amyloid beta and tau. The two positions are lightheartedly described as "ba-ptist" and "tau-ist" viewpoints in one scientific publication. Therein, it is suggested that "Tau-ists" believe that the tau protein abnormalities initiate the disease cascade, while "ba-ptists" believe that beta amyloid deposits are the causative factor in the disease.[18]

Tau hypothesis

The hypothesis that tau is the primary causative factor has long been grounded in the observation that deposition of amyloid plaques does not correlate well with neuron loss.[19] A mechanism for neurotoxicity has been proposed based on the loss of microtubule-stabilizing tau protein that leads to the degradation of the cytoskeleton.[20] However, consensus has not been reached on whether tau hyperphosphorylation precedes or is caused by the formation of the abnormal helical filament aggregates.[18] Support for the tau hypothesis also derives from the existence of other diseases known as tauopathies in which the same protein is identifiably misfolded.[21] However, a majority of researchers support the alternative hypothesis that amyloid is the primary causative agent.[18]

Amyloid hypothesis

The amyloid hypothesis is initially compelling because the gene for the amyloid beta precursor APP is located on chromosome 21, and patients with trisomy 21 - better known as Down syndrome - who have an extra gene copy exhibit AD-like disorders by 40 years of age.[22][23] The amyloid hypothesis points to the cytotoxicity of mature aggregated amyloid fibrils, which are believed to be the toxic form of the protein responsible for disrupting the cell's calcium ion homeostasis and thus inducing apoptosis.[24] This hypothesis is supported by the observation that higher levels of a variant of the beta amyloid protein known to form fibrils faster in vitro correlate with earlier onset and greater cognitive impairment in mouse models[25] and with AD diagnosis in humans.[26] However, mechanisms for the induced calcium influx, or proposals for alternative cytotoxic mechanisms, by mature fibrils are not obvious.

Flow chart depicting the role of apomorphine in Alzheimer's disease.

A more recent and broadly supported variation of the amyloid hypothesis identifies the cytotoxic species as an intermediate misfolded form of amyloid beta, neither a soluble monomer nor a mature aggregated polymer but an oligomeric species, possibly toroidal or star-shaped with a central channel[27] that may induce apoptosis by physically piercing the cell membrane.[28] This ion channel hypothesis postulates that oligomers of soluble, non-fibrillar Aβ form membrane ion channels allowing unregulated calcium influx into neurons.[29] A related alternative suggests that a globular oligomer localized to dendritic processes and axons in neurons is the cytotoxic species.[30][31] The prefibrillar aggregates were shown to be able to disrupt the membrane.[32]

The cytotoxic-fibril hypothesis presents a clear target for drug development: inhibit the fibrillization process. Much early development work on lead compounds has focused on this inhibition;[33][34][35] most are also reported to reduce neurotoxicity, but the toxic-oligomer theory would imply that prevention of oligomeric assembly is the more important process[36][37] [38] or that a better target lies upstream, for example in the inhibition of APP processing to amyloid beta.[39] For example, apomorphine was seen to significantly improve memory function through the increased successful completion of the Morris Water Maze.[36]

Soluble intracellular (o)Aβ42

Two papers have shown that oligomeric (o)Aβ42 (a species of Aβ), in soluble intracellular form, acutely inhibits synaptic transmission, a pathophysiology that characterizes AD (in its early stages), by activating casein kinase 2.[40][41]

Isoprenoid changes

A 1994 study [42] showed that the isoprenoid changes in Alzheimer's disease differ from those occurring during normal aging and that this disease cannot, therefore, be regarded as a result of premature aging. During aging the human brain shows a progressive increase in levels of dolichol, a reduction in levels of ubiquinone, but relatively unchanged concentrations of cholesterol and dolichyl phosphate. In Alzheimer's disease, the situation is reversed with decreased levels of dolichol and increased levels of ubiquinone. The concentrations of dolichyl phosphate are also increased, while cholesterol remains unchanged. The increase in the sugar carrier dolichyl phosphate may reflect an increased rate of glycosylation in the diseased brain and the increase in the endogenous anti-oxidant ubiquinone an attempt to protect the brain from oxidative stress, for instance induced by lipid peroxidation.[43] Ropren, identified previously in Russia, is neuroprotective in a rat model of Alzheimer's disease.[44][45]

Glucose consumption

The human brain is one of the most metabolically active organs in the body and metabolizes a large amount of glucose to produce cellular energy in the form of adenosine triphosphate (ATP).[46] Despite its high energy demands, the brain is relatively inflexible in its ability to utilize substrates for energy production and relies almost entirely on circulating glucose for its energy needs.[47] This dependence on glucose puts the brain at risk if the supply of glucose is interrupted, or if its ability to metabolize glucose becomes defective. If the brain is not able to produce ATP, synapses cannot be maintained and cells cannot function, ultimately leading to impaired cognition.[47]

Imaging studies have shown decreased utilization of glucose in the brains of Alzheimer’s disease patients early in the disease, before clinical signs of cognitive impairment occur. This decrease in glucose metabolism worsens as clinical symptoms develop and the disease progresses.[48][49] Studies have found a 17%-24% decline in cerebral glucose metabolism in patients with Alzheimer’s disease, compared with age-matched controls.[50] Numerous imaging studies have since confirmed this observation.

Abnormally low rates of cerebral glucose metabolism are found in a characteristic pattern in the Alzheimer’s disease brain, particularly in the posterior cingulate, parietal, temporal, and prefrontal cortices. These brain regions are believed to control multiple aspects of memory and cognition. This metabolic pattern is reproducible and has even been proposed as a diagnostic tool for Alzheimer’s disease. Moreover, diminished cerebral glucose metabolism (DCGM) correlates with plaque density and cognitive deficits in patients with more advanced disease.[50][51]

Diminished cerebral glucose metabolism (DCGM) may not be solely an artifact of brain cell loss since it occurs in asymptomatic patients at risk for Alzheimer’s disease, such as patients homozygous for the epsilon 4 variant of the apolipoprotein E gene (APOE4, a genetic risk factor for Alzheimer’s disease), as well as in inherited forms of Alzheimer’s disease.[52] Given that DCGM occurs before other clinical and pathological changes occur, it is unlikely to be due to the gross cell loss observed in Alzheimer’s disease.[47]

In imaging studies involving young adult APOE4 carriers, where there were no signs of cognitive impairment, diminished cerebral glucose metabolism (DCGM) was detected in the same areas of the brain as older subjects with Alzheimer’s disease.[52] However, DCGM is not exclusive to APOE4 carriers. By the time Alzheimer’s has been diagnosed, DCGM occurs in genotypes APOE3/E4, APOE3/E3, and APOE4/E4.[53] Thus, DCGM is a metabolic biomarker for the disease state.[54]

Insulin signaling

A connection has been established between Alzheimer's disease and diabetes during the past decade, as insulin resistance, which is a characteristic hallmark of diabetes, has also been observed in brains of subjects suffering from Alzheimer's disease.[55] Neurotoxic oligomeric amyloid-β species decrease the expression of insulin receptors on the neuronal cell surface[56] and abolish neuronal insulin signaling.[55] It has been suggested that neuronal gangliosides, which take part in the formation of membrane lipid microdomains, facilitate amyloid-β-induced removal of the insulin receptors from the neuronal surface.[57] In Alzheimer's disease, oligomeric amyloid-β species trigger TNF-α signaling.[55] c-Jun N-terminal kinase activation by TNF-α in turn activates stress-related kinases and results in IRS-1 serine phosphorylation, which subsequently blocks downstream insulin signaling.[55][58][59] The resulting insulin resistance contributes to cognitive impairment. Consequently, increasing neuronal insulin sensitivity and signaling may constitute a novel therapeutic approach to treat Alzheimer's disease.[60][61]

Oxidative stress

Oxidative stress is emerging as a key factor in the pathogenesis of AD.[62] Reactive oxygen species (ROS) over-production is thought to play a critical role in the accumulation and deposition of amyloid beta in AD.[63] Brains of AD patients have elevated levels of oxidative DNA damage in both nuclear and mitochondrial DNA, but the mitochondrial DNA has approximately 10-fold higher levels than nuclear DNA.[64] Aged mitochondria may be the critical factor in the origin of neurodegeneration in AD.[63] Even individuals with mild cognitive impairment, the phase between normal aging and early dementia, have increased oxidative damage in their nuclear and mitochondrial brain DNA[65] (see Aging brain).

Large gene instability hypothesis

A bioinformatics analysis in 2017[66] revealed that extremely large human genes are significantly over-expressed in brain and take part in the postsynaptic architecture. These genes are also highly enriched in cell adhesion Gene Ontology (GO) terms and often map to chromosomal fragile sites.[67] The majority of known Alzheimer's disease risk gene products including the amyloid precursor protein (APP) and gamma-secretase, as well as the APOE receptors and GWAS risk loci take part in similar cell adhesion mechanisms. It was concluded that dysfunction of cell and synaptic adhesion is central to Alzheimer's disease pathogenesis, and mutational instability of large synaptic adhesion genes may be the etiological trigger of neurotransmission disruption and synaptic loss in brain aging. As a typical example, this hypothesis explains the APOE risk locus of AD in context of signaling of its giant lipoprotein receptor, LRP1b which is a large tumor-suppressor gene with brain-specific expression and also maps to an unstable chromosomal fragile site. The large gene instability hypothesis puts the DNA damage mechanism at the center of Alzheimer's disease pathophysiology.

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