Proteinopathy | |
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Micrograph of a section of the cerebral cortex from a person with Alzheimer's disease, immunostained with an antibody to amyloid beta (brown), a protein fragment that accumulates in amyloid plaques and cerebral amyloid angiopathy. 10X microscope objective. |
In medicine, proteinopathy ([pref. protein]; -pathy [suff. disease]; proteinopathiespl.; proteinopathicadj), or proteopathy, protein conformational disorder, or protein misfolding disease, is a class of diseases in which certain proteins become structurally abnormal, and thereby disrupt the function of cells, tissues and organs of the body. [1] [2]
Often the proteins fail to fold into their normal configuration; in this misfolded state, the proteins can become toxic in some way (a toxic gain-of-function) or they can lose their normal function. [3] The proteinopathies include such diseases as Creutzfeldt–Jakob disease (and a variant associated with mad cow disease) and other prion diseases, Alzheimer's disease, Parkinson's disease, amyloidosis, multiple system atrophy, and a wide range of other disorders. [2] [4] [5] [6] [7] [8] The term proteopathy was first proposed in 2000 by Lary Walker and Harry LeVine. [1]
The concept of proteopathy can trace its origins to the mid-19th century, when, in 1854, Rudolf Virchow coined the term amyloid ("starch-like") to describe a substance in cerebral corpora amylacea that exhibited a chemical reaction resembling that of cellulose. In 1859, Friedreich and Kekulé demonstrated that, rather than consisting of cellulose, "amyloid" actually is rich in protein. [9] Subsequent research has shown that many different proteins can form amyloid, and that all amyloids show birefringence in cross-polarized light after staining with the dye Congo red, as well as a fibrillar ultrastructure when viewed with an electron microscope. [9] However, some proteinaceous lesions lack birefringence and contain few or no classical amyloid fibrils, such as the diffuse deposits of amyloid beta (Aβ) protein in the brains of people with Alzheimer's. [10] Furthermore, evidence has emerged that small, non-fibrillar protein aggregates known as oligomers are toxic to the cells of an affected organ, and that amyloidogenic proteins in their fibrillar form may be relatively benign. [11] [12]
In most, if not all proteinopathies, a change in the 3-dimensional folding conformation increases the tendency of a specific protein to bind to itself. [5] In this aggregated form, the protein is resistant to clearance and can interfere with the normal capacity of the affected organs. In some cases, misfolding of the protein results in a loss of its usual function. For example, cystic fibrosis is caused by a defective cystic fibrosis transmembrane conductance regulator (CFTR) protein, [3] and in amyotrophic lateral sclerosis / frontotemporal lobar degeneration (FTLD), certain gene-regulating proteins inappropriately aggregate in the cytoplasm, and thus are unable to perform their normal tasks within the nucleus. [13] [14]
Because proteins share a common structural feature known as the polypeptide backbone, all proteins have the potential to misfold under some circumstances. [15] However, only a relatively small number of proteins are linked to proteopathic disorders, possibly due to structural idiosyncrasies of the vulnerable proteins. For example, proteins that are normally unfolded or relatively unstable as monomers (that is, as single, unbound protein molecules) are more likely to misfold into an abnormal conformation. [5] [15] [16] In nearly all instances, the disease-causing molecular configuration involves an increase in beta-sheet secondary structure of the protein. [5] [15] [17] [18] [19]
The abnormal proteins in some proteopathies have been shown to fold into multiple 3-dimensional shapes; these variant, proteinaceous structures are defined by their different pathogenic, biochemical, and conformational properties. [20] They have been most thoroughly studied with regard to prion disease, and are referred to as protein strains. [21] [22]
The likelihood that proteinopathy will develop is increased by certain risk factors that promote the self-assembly of a protein. These include destabilizing changes in the primary amino acid sequence of the protein, post-translational modifications (such as hyperphosphorylation), changes in temperature or pH, an increase in production of a protein, or a decrease in its clearance. [1] [5] [15] Advancing age is a strong risk factor, [1] as is traumatic brain injury. [23] [24] In the aging brain, multiple proteopathies can overlap. [25] For example, in addition to tauopathy and Aβ-amyloidosis (which coexist as key pathologic features of Alzheimer's disease), many Alzheimer patients have concomitant synucleinopathy (Lewy bodies) in the brain. [26]
It is hypothesized that chaperones and co-chaperones (proteins that assist protein folding) may antagonize proteotoxicity during aging and in protein misfolding-diseases to maintain proteostasis. [27] [28] [29]
Some proteins can be induced to form abnormal assemblies by exposure to the same (or similar) protein that has folded into a disease-causing conformation, a process called 'seeding' or 'permissive templating'. [30] [31] In this way, the disease state can be brought about in a susceptible host by the introduction of diseased tissue extract from an affected donor. The best known forms of inducible proteopathy are prion diseases, [32] which can be transmitted by exposure of a host organism to purified prion protein in a disease-causing conformation. [33] [34]
There is now evidence that other proteinopathies can be induced by a similar mechanism, including Aβ amyloidosis, amyloid A (AA) amyloidosis, and apolipoprotein AII amyloidosis, [31] [35] tauopathy, [36] synucleinopathy, [37] [38] [39] [40] and the aggregation of superoxide dismutase-1 (SOD1), [41] [42] polyglutamine, [43] [44] and TAR DNA-binding protein-43 (TDP-43). [45]
In all of these instances, an aberrant form of the protein itself appears to be the pathogenic agent. In some cases, the deposition of one type of protein can be experimentally induced by aggregated assemblies of other proteins that are rich in β-sheet structure, possibly because of structural complementarity of the protein molecules. For example, AA amyloidosis can be stimulated in mice by such diverse macromolecules as silk, the yeast amyloid Sup35, and curli fibrils from the bacterium Escherichia coli . [46] AII amyloid can be induced in mice by a variety of β-sheet rich amyloid fibrils, [47] and cerebral tauopathy can be induced by brain extracts that are rich in aggregated Aβ. [48] There is also experimental evidence for cross-seeding between prion protein and Aβ. [49] In general, such heterologous seeding is less efficient than is seeding by a corrupted form of the same protein.
The development of effective treatments for many proteopathies has been challenging. [75] [76] Because the proteopathies often involve different proteins arising from different sources, treatment strategies must be customized to each disorder; however, general therapeutic approaches include maintaining the function of affected organs, reducing the formation of the disease-causing proteins, preventing the proteins from misfolding and/or aggregating, or promoting their removal. [77] [75] [78] For example, in Alzheimer's disease, researchers are seeking ways to reduce the production of the disease-associated protein Aβ by inhibiting the enzymes that free it from its parent protein. [76] Another strategy is to use antibodies to neutralize specific proteins by active or passive immunization. [79] In some proteopathies, inhibiting the toxic effects of protein oligomers might be beneficial. [80]
For example, Amyloid A (AA) amyloidosis can be reduced by treating the inflammatory state that increases the amount of the protein in the blood (referred to as serum amyloid A, or SAA). [75] In immunoglobulin light chain amyloidosis (AL amyloidosis), chemotherapy can be used to lower the number of the blood cells that make the light chain protein that forms amyloid in various bodily organs. [81] Transthyretin (TTR) amyloidosis (ATTR) results from the deposition of misfolded TTR in multiple organs. [82] Because TTR is mainly produced in the liver, TTR amyloidosis can be slowed in some hereditary cases by liver transplantation. [83] TTR amyloidosis also can be treated by stabilizing the normal assemblies of the protein (called tetramers because they consist of four TTR molecules bound together). Stabilization prevents individual TTR molecules from escaping, misfolding, and aggregating into amyloid. [84] [85]
Several other treatment strategies for proteopathies are being investigated, including small molecules and biologic medicines such as small interfering RNAs, antisense oligonucleotides, peptides, and engineered immune cells. [84] [81] [86] [87] In some cases, multiple therapeutic agents may be combined to improve effectiveness. [81] [88]
A prion is a misfolded protein that induces misfolding in normal variants of the same protein, leading to cellular death. Prions are responsible for prion diseases, known as transmissible spongiform encephalopathy (TSEs), which are fatal and transmissible neurodegenerative diseases affecting both humans and animals. These proteins can misfold sporadically, due to genetic mutations, or by exposure to an already misfolded protein, leading to an abnormal three-dimensional structure that can propagate misfolding in other proteins.
Amyloids are aggregates of proteins characterised by a fibrillar morphology of typically 7–13 nm in diameter, a β-sheet secondary structure and ability to be stained by particular dyes, such as Congo red. In the human body, amyloids have been linked to the development of various diseases. Pathogenic amyloids form when previously healthy proteins lose their normal structure and physiological functions (misfolding) and form fibrous deposits within and around cells. These protein misfolding and deposition processes disrupt the healthy function of tissues and organs.
Alpha-synuclein (aSyn) is a protein that, in humans, is encoded by the SNCA gene. Alpha-synuclein is a neuronal protein that regulates synaptic vesicle trafficking and subsequent neurotransmitter release.
Amyloidosis is a group of diseases in which abnormal proteins, known as amyloid fibrils, build up in tissue. There are several non-specific and vague signs and symptoms associated with amyloidosis. These include fatigue, peripheral edema, weight loss, shortness of breath, palpitations, and feeling faint with standing. In AL amyloidosis, specific indicators can include enlargement of the tongue and periorbital purpura. In wild-type ATTR amyloidosis, non-cardiac symptoms include: bilateral carpal tunnel syndrome, lumbar spinal stenosis, biceps tendon rupture, small fiber neuropathy, and autonomic dysfunction.
Transthyretin (TTR or TBPA) is a transport protein in the plasma and cerebrospinal fluid that transports the thyroid hormone thyroxine (T4) and retinol to the liver. This is how transthyretin gained its name: transports thyroxine and retinol. The liver secretes TTR into the blood, and the choroid plexus secretes TTR into the cerebrospinal fluid.
The tau proteins form 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 beta denotes peptides of 36–43 amino acids that are the main component of the amyloid plaques found in the brains of people with Alzheimer's disease. The peptides derive from the amyloid-beta precursor protein (APP), which is cleaved by beta secretase and gamma secretase to yield Aβ in a cholesterol-dependent process and substrate presentation. Both neurons and oligodendrocytes produce and release Aβ in the brain, contributing to formation of amyloid plaques. Aβ molecules can aggregate to form flexible soluble oligomers which may exist in several forms. It is now believed that certain misfolded oligomers can induce other Aβ molecules to also take the misfolded oligomeric form, leading to a chain reaction akin to a prion infection. The oligomers are toxic to nerve cells. The other protein implicated in Alzheimer's disease, tau protein, also forms such prion-like misfolded oligomers, and there is some evidence that misfolded Aβ can induce tau to misfold.
Amyloid plaques are extracellular deposits of amyloid beta (Aβ) protein that present 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. 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. Plaques form when Aβ misfolds and aggregates into oligomers and longer polymers, the latter of which are characteristic of amyloid.
The major prion protein (PrP) is encoded in the human body by the PRNP gene also known as CD230. Expression of the protein is most predominant in the nervous system but occurs in many other tissues throughout the body.
Tauopathies are a class of neurodegenerative diseases characterized by the aggregation of abnormal tau protein. Hyperphosphorylation of tau proteins causes them to dissociate from microtubules and form insoluble aggregates called neurofibrillary tangles. Various neuropathologic phenotypes have been described based on the anatomical regions and cell types involved as well as the unique tau isoforms making up these deposits. The designation 'primary tauopathy' is assigned to disorders where the predominant feature is the deposition of tau protein. Alternatively, diseases exhibiting tau pathologies attributed to different and varied underlying causes are termed 'secondary tauopathies'. Some neuropathologic phenotypes involving tau protein are Alzheimer's disease, frontotemporal dementia, progressive supranuclear palsy, and corticobasal degeneration.
A neurodegenerative disease is caused by the progressive loss of neurons, in the process known as neurodegeneration. Neuronal damage may also ultimately result in their death. Neurodegenerative diseases include amyotrophic lateral sclerosis, multiple sclerosis, Parkinson's disease, Alzheimer's disease, Huntington's disease, multiple system atrophy, tauopathies, and prion diseases. Neurodegeneration can be found in the brain at many different levels of neuronal circuitry, ranging from molecular to systemic. Because there is no known way to reverse the progressive degeneration of neurons, these diseases are considered to be incurable; however research has shown that the two major contributing factors to neurodegeneration are oxidative stress and inflammation. Biomedical research has revealed many similarities between these diseases at the subcellular level, including atypical protein assemblies and induced cell death. These similarities suggest that therapeutic advances against one neurodegenerative disease might ameliorate other diseases as well.
Karen K. Hsiao Ashe is a professor at the Department of Neurology and Neuroscience at the University of Minnesota (UMN) Medical School, where she holds the Edmund Wallace and Anne Marie Tulloch Chairs in Neurology and Neuroscience. She is the founding director of the N. Bud Grossman Center for Memory Research and Care, and her specific research interest is memory loss resulting from Alzheimer's disease and related dementias. Her research has included the development of an animal model of Alzheimer's.
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.
Cardiac amyloidosis is a subcategory of amyloidosis where there is depositing of the protein amyloid in the cardiac muscle and surrounding tissues. Amyloid, a misfolded and insoluble protein, can become a deposit in the heart's atria, valves, or ventricles. These deposits can cause thickening of different sections of the heart, leading to decreased cardiac function. The overall decrease in cardiac function leads to a plethora of symptoms. This multisystem disease was often misdiagnosed, with a corrected analysis only during autopsy. Advancements of technologies have increased earlier accuracy of diagnosis. Cardiac amyloidosis has multiple sub-types including light chain, familial, and senile. One of the most studied types is light chain cardiac amyloidosis. Prognosis depends on the extent of the deposits in the body and the type of amyloidosis. New treatment methods are actively being researched in regards to the treatment of heart failure and specific cardiac amyloidosis problems.
The familial amyloid neuropathies are a rare group of autosomal dominant diseases wherein the autonomic nervous system and/or other nerves are compromised by protein aggregation and/or amyloid fibril formation.
The ion channel hypothesis of Alzheimer's disease (AD), also known as the channel hypothesis or the amyloid beta ion channel hypothesis, is a more recent variant of the amyloid hypothesis of AD, which identifies amyloid beta (Aβ) as the underlying cause of neurotoxicity seen in AD. While the traditional formulation of the amyloid hypothesis pinpoints insoluble, fibrillar aggregates of Aβ as the basis of disruption of calcium ion homeostasis and subsequent apoptosis in AD, the ion channel hypothesis in 1993 introduced the possibility of an ion-channel-forming oligomer of soluble, non-fibrillar Aβ as the cytotoxic species allowing unregulated calcium influx into neurons in AD.
Mathias Jucker is a Swiss neuroscientist, Professor, and a Director at the Hertie Institute for Clinical Brain Research of the University of Tübingen. He is also a group leader at the German Center for Neurodegenerative Diseases in Tübingen. Jucker is known for his research on the basic biologic mechanisms underlying brain aging and Alzheimer's disease.
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.
Hilal Lashuel is an American-Yemeni neuroscientist and chemist, currently an associate professor at the EPFL. His research focuses on protein misfolding and aggregation in the pathogenesis of Alzheimer's and Parkinson's diseases.
Li Gan is a neuroscientist and professor at Weill Cornell Medical College. She is known for her discovery of pathogenic tau protein acetylation in tauopathies and mechanisms of microglia dysfunction in neurodegeneration.