APM022 Pathophysiological Principles for Advanced Practice

Proteins involved in Alzheimer’s disease

The most prevalent neurodegenerative disease affecting approximately 4 million people in the United States of America is Alzheimer’s Disease (AD). AD is mostly prevalent among the elderly population and number of Alzheimer’s cases is likely to increase by 100 million. AD is characterized by progressive decline of various cognitive functions, like memory impairment, loss of ability to carry out sentence structuring, inability to carry out complex tasks requiring muscle coordination, inability to recognize and subsequently use any familiar objects, inability to plan, organize, and carry out normal activities. The leading cause of Dementia is AD. The most common features of the disease are senile plaques and neurofibrillary tangles (NFTs).

The other common features of AD include amyloid plaques, gliosis, dendritic or neuronal loss, dystrophic neuritis, neuropil threads, cerebrovascular amyloids, Hirano bodies, granulovacuolar degeneration, synapse loss, among others. Mainly 2 proteins are involved in the development of AD. These include the β-amyloid peptide and the Tau protein that remains associated with microtubules. The molecular mechanisms leading to the AD involves the formation of the Amyloid β peptide, followed by its clustering into amyloid plaques, also called senile plaques that develops on blood vessels. The plaques are characterized by spherical extracellular lesions 10-200 mm in diameter. The specific species of proteins that comprises the amyloid is the Amyloid β, which is a product of amyloid precursor proteins (APP) metabolism. The neuritic senile plaques consist of a central nucleus made of 6- to 10-nm filaments of Amyloid β protein, arranged in bundles that radiates from the centre. Surrounding the core of the plaques is an argyrophilic rim of dystrophic synapses and neurites (particularly axons) along with altered membranes and paired regions of helical filaments.

The APPs undergoes cleavage by BACE and gamma secretase, which can result in the formation of a 42-amino acid insoluble amyloid β peptide that gives rise to the formation of insoluble plaques. Mutation in either presenilin 1 or 2 enhance the activity of gamma-secretase towards the production of the toxic 42 peptides Amyloid β. Amyloid oligomers are the most toxic forms of amyloid-β. This excess amyloid-β oligomerizes to form fibrils and results in amyloid plaques. The amyloid-β and associated plaque proteins are toxic to the neurons. This leads to synaptic loss, production of NFTs, neuronal death and Alzheimer’s disease. Some of the toxic oligomeric assemblies of amyloid- β include the protofibril, annular assemblies, amyloid- β derived diffusible ligands, among others.

Amyloid plaque formation related death of neurons involves two distinct processes. These include the formation of NFTs and oxidative damage or inflammation. Substances that are required for proper functioning of the neurons are transferred from the cell body via the microtubules of the neurons. The Tau protein is responsible for the structural integrity of microtubules. In the case of Alzheimer’s disease, the Tau protein becomes hyperphosphorylated. Thus, instead of binding to the microtubules, they bind to one another forming oligomers as well as large filamentous aggregates. Misfolding and hyperphosphorylation of Tau results in bundling and subsequent stabilization of actin filaments, which in turn gives rise to elongated, dysfunctional mitochondria and ultimately resulting in oxidative stress. Subsequent oxidative stress or absence of nuclear REST (repressor element 1-silenciing transcription factor) results in DNA damage, which activates the heterochromatin loss and subsequent apoptosis. It also causes axonal transport defects and excitotoxicity. Inflammation experienced in AD is carried out by astrocytes, which activated releases prostaglandins or arachidonic acid, which results in inflammation. Moreover, microglial cells give rise to free radicals that have a damaging effect on neurons resulting in the death of neurons.

Proteins can carry out their designated functions only if they are properly folded. They undergo folding with or without the help of chaperones. Misfolded proteins form large insoluble aggregates and is involved in the pathogenesis of AD. These toxic aggregates then interact with other proteins, initiate a toxic cycle that eventually results in loss of cellular function, and ultimately cell death. Accumulation of misfolded proteins and subsequent decrease in their clearance results in amyloid diseases such as AD. Apolipoprotein E is another protein involved in the progression of AD. Apolipoprotein E is involved in the transport of cholesterol to the central nervous system. There are 3 apoE polymorphic alleles, namely APOE 2, 3 and 4, of which APOE4 is the most important risk factor associated with late onset of AD. Presence of the APOE4 allele results in decreased clearance of the amyloid β from the brain. The APOE4 allele or carrier status is associated with increased deposition of amyloid β plaques. It is also associated with increased deposition of cerebro-vascular amyloid β. This increases the risk of developing cerebral amyloid angiopathy.

Cellular prion proteins or PrP^C is involved in the development of Prion diseases. The cellular prion proteins play an important role in the propagation of protinaceous infectious agents called Prions, which in turn gives rise to neurodegenerative diseases. The cellular prion protein acts as a cell surface receptor for the amyloid β oligomers. The cellular prion proteins transduce the neurotoxic signals of the amyloid β oligomers, resulting in synaptic failure and subsequent cognitive impairment, which are the important characteristics of AD. Amyloid β and Tau proteins have been shown to possess prion protein like functions, that enables them to spread throughout the regions of the brain. This is because, the misfolded amyloid β and Tau proteins can initiate a seeding effect, resulting in interactions with normal amyloid β and Tau proteins, thereby inducing them to undergo misfolding. Such misfolded proteins then spread to the surrounding regions of the brain initiating a toxic effect. The ubiquitin proteasome complex also plays an essential role in the development of AD. Misfolded proteins are targeted for degradation by the ubiquitin proteasome complex by covalent binding of ubiquitin to the substrate protein. This ubiquitin bound protein is then subjected to degradation involving a cascade of enzymes. The ubiquitin proteasome system is linked to phases of AD characterized by synaptic dysfunction and neurodegeneration. Malfunctioning or overloading of the ubiquitin proteasome complex prevents degradation of the misfolded proteins resulting in the case of AD. Moreover, misfolding of the proteins, results in structural changes, which are not recognized by the ubiquitin proteasome system, thereby preventing degradation.

Experimental approaches for studying Alzheimer’s disease

Various experimental approaches are present for studying of AD. These include positron emission tomography (PET), single proton emission computerized tomography (SPECT), functional magnetic resonance imaging (MRI), among others. Non-invasive imaging of the cerebral amyloid deposits by PET scanning is a tool for diagnosis of AD. It involves the use of a radiolabeled precursor and glucose. PET scanning with the ¹¹C-labeled Pittsburgh Compound B (PiB), a benzothiazole derivative of Thioflavin T, is a non-invasive method to detect the presence of cerebral amyloid. PET scan using florbetapir, reveal moderate to frequent build-up of fibril-containing plaques, which is a cardinal feature of the disease. PET scanning of in vivo microgliosis reveals the presence of elevated reactivity in AD patients.

PET and SPECT scans use imaging probes that are radioactively labeled and these probes then carry out non-invasive imaging of the amyloid β plaques and the NFTs. Recent research has developed a number of radioactive probes that are capable of detecting Tau protein aggregates with the help of SPECT scanning. Functional MRI is also used in the detection of AD. Functional MRI provides information about hippocampal atrophy. Other experimental approaches used for the study of AD involve transgenic mice over expressing amyloid precursor proteins derived from humans. In vivo studies are also carried out to determine the molecular mechanisms that give rise to the development of AD.