Primary age-related tauopathy (PART) is a neuropathological designation introduced in 2014 to describe the neurofibrillary tangles (NFT) that are commonly observed in the brains of normally aged and cognitively impaired individuals that can occur independently of the amyloid plaques of Alzheimer's disease (AD). [1] [2] The term and diagnostic criteria for PART were developed by a large group of neuropathologists, spearheaded by Drs. John F. Crary (then at Columbia University Medical Center) and Peter T. Nelson (University of Kentucky). [3] Despite some controversy, [4] [5] the term PART has been widely adopted, with the consensus criteria cited over 1130 times as of April 2023 according to Google Scholar.
At autopsy, the hallmark of PART is the presence of Alzheimer-type neurofibrillary tangles (NFTs) composed of abnormal tau protein in neurons in the medial temporal lobe, but no amyloid-beta (Aβ42) peptide accumulation in plaques. [2] This ultimately leads to neuronal death and brain atrophy. [6] 18% of Alzheimer neuropathological changes in cognitively normal and 5% of cognitively impaired elderly cases have been shown to display this pattern of degeneration. [7] Patients with severe PART typically exhibit mild cognitive impairment or an amnestic dementia. [2]
Patients with PART display neurofibrillary tangles that are essentially identical to those occurring in mild to moderate-stage Alzheimer's disease and other tauopathies. [8] Amyloid pathology is sparse or absent in patients with PART. [2] [7] If few senile plaques are found, Thal phase grading can be implemented to differentiate the pathology as either PART or AD. [7]
Patients with PART can be cognitively normal, mildly cognitively impaired, or demented. [9] [7] Specifically, higher stages of tangle burden (i.e. Braak III or IV) in PART have been found to be associated with more rapid decline on tasks involving episodic and semantic memory along with tests of processing speed and attention. [9] Braak state 0 is restricted to the cortex, state l-ll bound by transentorhinal region and it can progress into limbic region of the brain (stage lll-lV). [10] PART can be further categorized as symptomatic (cognitive impairment and dementia) and asymptomatic (no signs of dementia). [7] [6] One current hypothesis suggests that PART related dementia could be infrequent in younger populations, but may show symptomatic onset within oldest old (people greater than 90 years old). [11] Given that the elderly represent a fast growing segment of the population worldwide, further research is needed to understand how PART related pathological process can manifest in specific clinical symptoms.
Furthermore, serological testing cannot be used to identify PART patients and MRI scans are the only current available diagnostic tools. [12]
Given the similarities in the pattern of neurofibrillary tangles in PART, some scientists have argued that they represent the same phenomenon. [13] [14] However, others have argued that sufficient evidence exists to conclude that PART represents a pathological process. [15] Further more, Aβ42 presence in AD contributes to tau hyper phosphorylation and consequently its development into NFTs. [16] [17] Aβ42 is absent in PART and due to several mechanisms underlying tau formation and maintenance, it would be necessary to separate PART from AD due to implications with respect to developing diagnostics and therapeutics. [2] [18] [19]
PART has been associated with microtubule association tau protein (MAPT) H1 haplotype and no association has been seen with APOE ε4, a gene strongly linked to AD. [2] [8] [19] Thus another piece of evidence supporting the hypothesis that PART represents a novel diagnostic category. Also transformation as a result of tau mutation into isoforms (3R and 4R) on chromosome 17 has been linked to Parkinson and frontotemporal dementia. [18] Transformation in tau gene on chromosome 17 can be linked to PART due to the fact that the tau protein analyzed from PART NFTs consist of 3R and 4R isoforms. [12] MAPT gene results in different tau protein isoforms due to splicing patterns of exon 10. [10]
MRI, immunoblot and immunofluorescence tools have allowed researchers to identify and observe tau protein aggregation both intra and intercellullary as well as their interaction with other proteins. [2] [7] [18] Immunohistochemistry of brain autopsy of PART cases reveal that NFT's appear in the hippocampus area which is involved in memory formation. [2] Also development of specific tau tracers allows for positron emission tomography (PET) imaging allowing for both intracellular and extracellular observation of tau protein behaviours. [10]
Owing to these imaging and staining advancements, tau has been identified to be associated with microtubules in neuron cells. [20] Tau protein stabilize the microtubules and are involved in fast dendrite growth, retrograde and antegrade transport intracellularly and neuron maintenance. [20]
Tau protein is divided into three segments, i.e. N-terminal (regulated spacing of microtubules), C-terminal (microtubule polymerization), proline rich domain (kinase) and microtubule binding domain. [12] Tau in brain of patients with tauopathies is hyperphosphorylated which causes the tau protein to dissociate from the microtubule then aggregate in to β-plated sheet arrangement. [12] PART cases is due to tau protein isoforms (3 and 4 microtubule binding repeats) abnormal ration inneural cells resulting in their self assembling and accumulation resulting in NFT formation in brain. [12] [17] [18] Detachment of tau from microtubules causes the neuron to lose its ability to sustain its self and thus ultimately loses function.
Hyper-phosphorylation of tau protein was initially thought to be caused by Aβ42 but since PART cases generally lack senile plaques, other causes were investigated. One such cause was found to me the microtubule affinity-regulating kinase (MARK) since it is involved in tau phosphorylation and dephosphorylation. [18] [19] Overexpression of MARK gene results in excessive tau phosphorylation and eventually NFT's formation. [18] [19] It is suspected that at a threonine (Thr175 ), kinases such as GSK3β and MARK hyper-phosphorylate the mutant tau protein residue. [20] Also researcher have linked to NFT survive (mutant tau protein maintenance) to heat shock protein 90 (HSP90) since HSP90 performs such function in cancer cells and its inhibition resulted in elimination of tau aggregates in vivo. [18]
Recent finding's on tau regulation has revealed small non coding RNAs bind to recognition motifs on mRNAs and silence their expression through post-translational regulation. [21] MiRNA-219 binds to tau mRNA and represses tau protein synthesis since in fly model brain, over expression of miRNA-219 reduced the tau protein accumulation. [21]
These options listed below have not been yet linked or specified for PART treatment since the disease is yet to gain acceptance as a unique abnormality case by the medical community. Thus the treatments represent possible future options as per the research and finding in the medical literature.
PU-DZ8 can pass the blood brain barrier and act on HSP90 to inhibit it. [18]
Staurosporine, Methylene blue and other kinase inhibitors can pass the blood brain barrier and inhibit MARK which consequently down regulation tau protein hyper-phosphorylation and ultimately its detachment from microtubules. [19]
Lewy bodies are the inclusion bodies – abnormal aggregations of protein – that develop inside nerve cells affected by Parkinson's disease (PD), the Lewy body dementias, and some other disorders. They are also seen in cases of multiple system atrophy, particularly the parkinsonian variant (MSA-P).
Progressive supranuclear palsy (PSP) is a late-onset degenerative disease involving the gradual deterioration and death of specific volumes of the brain. The condition leads to symptoms including loss of balance, slowing of movement, difficulty moving the eyes, and cognitive impairment. PSP may be mistaken for other neurodegenerative diseases such as Parkinson's, frontotemporal dementia and Alzheimer's. The cause of the condition is uncertain, but involves accumulation of tau protein within the brain. Medications such as levodopa and amantadine may be useful in some cases.
The tau proteins are a group of six highly soluble protein isoforms produced by alternative splicing from the gene MAPT. They have roles primarily in maintaining the stability of microtubules in axons and are abundant in the neurons of the central nervous system (CNS), where the cerebral cortex has the highest abundance. They are less common elsewhere but are also expressed at very low levels in CNS astrocytes and oligodendrocytes.
Amyloid plaques are extracellular deposits of the amyloid beta (Aβ) protein mainly in the grey matter of the brain. Degenerative neuronal elements and an abundance of microglia and astrocytes can be associated with amyloid plaques. Some plaques occur in the brain as a result of aging, but large numbers of plaques and neurofibrillary tangles are characteristic features of Alzheimer's disease. Abnormal neurites in amyloid plaques are tortuous, often swollen axons and dendrites. The neurites contain a variety of organelles and cellular debris, and many of them include characteristic paired helical filaments, the ultrastructural component of neurofibrillary tangles. The plaques are highly variable in shape and size; in tissue sections immunostained for Aβ, they comprise a log-normal size distribution curve with an average plaque area of 400-450 square micrometers (µm²). The smallest plaques, which often consist of diffuse deposits of Aβ, are particularly numerous. The apparent size of plaques is influenced by the type of stain used to detect them, and by the plane through which they are sectioned for analysis under the microscope. Plaques form when Aβ misfolds and aggregates into oligomers and longer polymers, the latter of which are characteristic of amyloid. Misfolded and aggregated Aβ is thought to be neurotoxic, especially in its oligomeric state.
Frontotemporal lobar degeneration (FTLD) is a pathological process that occurs in frontotemporal dementia. It is characterized by atrophy in the frontal lobe and temporal lobe of the brain, with sparing of the parietal and occipital lobes.
Neurofibrillary tangles (NFTs) are aggregates of hyperphosphorylated tau protein that are most commonly known as a primary biomarker of Alzheimer's disease. Their presence is also found in numerous other diseases known as tauopathies. Little is known about their exact relationship to the different pathologies.
Pittsburgh compound B (PiB) is a radioactive analog of thioflavin T, which can be used in positron emission tomography scans to image beta-amyloid plaques in neuronal tissue. Due to this property, Pittsburgh compound B may be used in investigational studies of Alzheimer's disease.
Corticobasal degeneration (CBD) is a rare neurodegenerative disease involving the cerebral cortex and the basal ganglia. CBD symptoms typically begin in people from 50 to 70 years of age, and the average disease duration is six years. It is characterized by marked disorders in movement and cognition, and is classified as one of the Parkinson plus syndromes. Diagnosis is difficult, as symptoms are often similar to those of other disorders, such as Parkinson's disease, progressive supranuclear palsy, and dementia with Lewy bodies, and a definitive diagnosis of CBD can only be made upon neuropathologic examination.
Tauopathy belongs to a class of neurodegenerative diseases involving the aggregation of tau protein into neurofibrillary or gliofibrillary tangles in the human brain. Tangles are formed by hyperphosphorylation of the microtubule protein known as tau, causing the protein to dissociate from microtubules and form insoluble aggregates. The mechanism of tangle formation is not well understood, and whether tangles are a primary cause of Alzheimer's disease or play a peripheral role is unknown.
The biochemistry of Alzheimer's disease, the most common cause of dementia, is not yet very well understood. Alzheimer's disease (AD) has been identified as a proteopathy: a protein misfolding disease due to the accumulation of abnormally folded amyloid beta (Aβ) protein in the brain. Amyloid beta is a short peptide that is an abnormal proteolytic byproduct of the transmembrane protein amyloid-beta precursor protein (APP), whose function is unclear but thought to be involved in neuronal development. The presenilins are components of proteolytic complex involved in APP processing and degradation.
Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease linked to repeated trauma to the head. The encephalopathy symptoms can include behavioral problems, mood problems, and problems with thinking. The disease often gets worse over time and can result in dementia. It is unclear if the risk of suicide is altered.
Transmembrane protein 106B is a protein that is encoded by the TMEM106B gene. It is found primarily within neurons and oligodendrocytes in the central nervous system with its subcellular location being in lysosomal membranes. TMEM106B helps facilitate important functions for maintaining a healthy lysosome, and therefore certain mutations and polymorphisms can lead to issues with proper lysosomal function. Lysosomes are in charge of clearing out mis-folded proteins and other debris, and thus, play an important role in neurodegenerative diseases that are driven by the accumulation of various mis-folded proteins and aggregates. Due to its impact on lysosomal function, TMEM106B has been investigated and found to be associated to multiple neurodegenerative diseases.
Heiko Braak is a German anatomist. Braak was born in Kiel, Schleswig-Holstein, and studied medicine at the universities of Hamburg, Berlin, and Kiel. He was Professor at the Institute of Clinical Neuroanatomy, Johann Wolfgang Goethe-University, Frankfurt am Main. Currently he is based at the 'Clinical Neuroanatomy Section, Department of Neurology, Center for Biomedical Research, University of Ulm, Germany.
Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems with language, disorientation, mood swings, loss of motivation, self-neglect, and behavioral issues. As a person's condition declines, they often withdraw from family and society. Gradually, bodily functions are lost, ultimately leading to death. Although the speed of progression can vary, the typical life expectancy following diagnosis is three to nine years.
Florbetaben, a fluorine-18 (18F)-labeled stilbene derivative, trade name NeuraCeq, is a diagnostic radiotracer developed for routine clinical application to visualize β-amyloid plaques in the brain. It is indicated for Positron Emission Tomography (PET) imaging of β-amyloid neuritic plaque density in the brains of adult patients with cognitive impairment who are being evaluated for Alzheimer's disease (AD) and other causes of cognitive impairment. β-amyloid is a key neuropathological hallmark of AD, so markers of β-amyloid plaque accumulation in the brain are useful in distinguishing AD from other causes of dementia. The tracer successfully completed a global multicenter phase 0–III development program and obtained approval in Europe, US and South Korea in 2014.
LATE is a term that describes a prevalent condition with impaired memory and thinking in advanced age, often culminating in the dementia clinical syndrome. In other words, the symptoms of LATE are similar to those of Alzheimer's disease.
Eva Braak (1939-2000) was a German anatomist, mostly known for the Braak and Braak Alzheimer disease stages. She was professor at the Institute of Clinical Neuroanatomy, Johann Wolfgang Goethe-University, Frankfurt am Main.
Lary Walker is an American neuroscientist and researcher at Emory University in Atlanta, Georgia. He is Associate Director of the Goizueta Alzheimer's Disease Research Center at Emory, and he is known for his research on the role of abnormal proteins in the causation of Alzheimer’s disease.
Granulovacuolar degeneration refers to the occurrence within neurons of abnormal, fluid-filled bubbles (vacuoles) containing a dense proteinaceous granule. Granulovacuoles occur most commonly in pyramidal neurons of the hippocampus. They are present in small numbers in non-demented elderly people, but increase in frequency in Alzheimer's disease and other tauopathies. In Alzheimer's disease, granulovacuoles proliferate stage-wise in different brain areas, and their prevalence is correlated with the degree of tauopathy, Abeta plaque pathology, and cerebral amyloid angiopathy. Immunohistochemical analyses have found that the inner granule includes several proteins, including tubulin tau protein, TDP-43 and others. Although granulovacuoles and their functional significance are still poorly understood, they have been compared to autophagic vacuoles
Alzheimer's disease (AD) in the Hispanic/Latino population is becoming a topic of interest in AD research as Hispanics and Latinos are disproportionately affected by Alzheimer's Disease and underrepresented in clinical research. AD is a neurodegenerative disease, characterized by the presence of amyloid-beta plaques and neurofibrillary tangles, that causes memory loss and cognitive decline in its patients. However, pathology and symptoms have been shown to manifest differently in Hispanic/Latinos, as different neuroinflammatory markers are expressed and cognitive decline is more pronounced. Additionally, there is a large genetic component of AD, with mutations in the amyloid precursor protein (APP), Apolipoprotein E APOE), presenilin 1 (PSEN1), bridging Integrator 1 (BIN1), SORL1, and Clusterin (CLU) genes increasing one's risk to develop the condition. However, research has shown these high-risk genes have a different effect on Hispanics and Latinos then they do in other racial and ethnic groups. Additionally, this population experiences higher rates of comorbidities, that increase their risk of developing AD. Hispanics and Latinos also face socioeconomic and cultural factors, such as low income and a language barrier, that affect their ability to engage in clinical trials and receive proper care.