In eukaryotic cells, an aggresome refers to an aggregation of
misfolded proteins
Protein folding is the physical process by which a protein chain is translated to its native three-dimensional structure, typically a "folded" conformation by which the protein becomes biologically functional. Via an expeditious and reproduci ...
in the cell, formed when the
protein degradation
Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called proteases, ...
system of the cell is overwhelmed. Aggresome formation is a highly regulated process that possibly serves to organize misfolded proteins into a single location.
Biogenesis
Correct
folding
Fold, folding or foldable may refer to:
Arts, entertainment, and media
* ''Fold'' (album), the debut release by Australian rock band Epicure
*Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot
*Above ...
requires
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s to assume one particular structure from a
constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the e ...
of possible but incorrect conformations. The failure of
polypeptides
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
A p ...
to adopt their proper structure is a major threat to cell function and
viability. Consequently, elaborate systems have evolved to protect cells from the deleterious effects of
misfolded proteins
Protein folding is the physical process by which a protein chain is translated to its native three-dimensional structure, typically a "folded" conformation by which the protein becomes biologically functional. Via an expeditious and reproduci ...
.
Cells mainly deploy three mechanisms to counteract misfolded proteins: up-regulating
chaperones to assist protein refolding,
proteolytic
Proteolysis is the breakdown of proteins into smaller polypeptides or amino acids. Uncatalysed, the hydrolysis of peptide bonds is extremely slow, taking hundreds of years. Proteolysis is typically catalysed by cellular enzymes called proteases, ...
degradation of the misfolded/damaged proteins involving ubiquitin–proteasome and autophagy–lysosome systems, and formation of detergent-insoluble aggresomes by transporting the misfolded proteins along
microtubules
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
to a region near the
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucle ...
. Intracellular deposition of misfolded protein aggregates into ubiquitin-rich
cytoplasmic
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. Th ...
inclusions is linked to the
pathogenesis
Pathogenesis is the process by which a disease or disorder develops. It can include factors which contribute not only to the onset of the disease or disorder, but also to its progression and maintenance. The word comes from Greek πάθος ''pat ...
of many diseases. Functional blockade of either degradative system leads to an enhanced aggresome formation. Why these aggregates form despite the existence of cellular machinery to recognize and degrade misfolded protein, and how they are delivered to cytoplasmic inclusions, are not known.
Aggresome formation is accompanied by redistribution of the intermediate filament protein vimentin to form a cage surrounding a pericentriolar core of aggregated, ubiquitinated protein. Disruption of microtubules blocks the formation of aggresomes. Similarly, inhibition of proteasome function also prevents the degradation of unassembled
presenilin-1
Presenilin-1 (PS-1) is a presenilin protein that in humans is encoded by the ''PSEN1'' gene. Presenilin-1 is one of the four core proteins in the gamma secretase complex, which is considered to play an important role in generation of amyloid beta ( ...
(PSE
1) molecules leading to their aggregation and deposition in aggresomes. Aggresome formation is a general response of cells which occurs when the capacity of the proteasome is exceeded by the production of aggregation-prone misfolded proteins.
Typically, an aggresome forms in response to a cellular stress which generates a large amount of misfolded or partially
denatured protein
In biochemistry, denaturation is a process in which proteins or nucleic acids lose the quaternary structure, tertiary structure, and secondary structure which is present in their native state, by application of some external stress or compound ...
:
hyperthermia
Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extreme ...
, overexpression of an insoluble or
mutant protein, etc. The formation of the aggresome is largely believed to be a protective response, sequestering potentially
cytotoxic
Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are an immune cell or some types of venom, e.g. from the puff adder (''Bitis arietans'') or brown recluse spider (''Loxosceles reclusa'').
Cell physiology
Treating cells ...
aggregates and also acting as a staging center for eventual autophagic clearance from the cell.
An aggresome forms around the
microtubule organizing center The microtubule-organizing center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spindl ...
in
eukaryotic
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
cells, adjacent to or enveloping the cell's
centrosome
In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progres ...
s.
Polyubiquitination
Ubiquitin is a small (8.6 kDa) regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ''ubiquitously''. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Fou ...
tags the protein for retrograde transport via HDAC6 binding and microtubule-based motor protein,
dynein
Dyneins are a family of cytoskeletal motor proteins that move along microtubules in cells. They convert the chemical energy stored in ATP to mechanical work. Dynein transports various cellular cargos, provides forces and displacements importa ...
. Moreover, substrates can also be targeted to the aggresome by a ubiquitin-independent pathway mediated by the stress-induced co-chaperone
BAG3
BAG family molecular chaperone regulator 3 is a protein that in humans is encoded by the ''BAG3'' gene. BAG3 is involved in chaperone-assisted selective autophagy.
Function
BAG proteins compete with Hip-1 for binding to the Hsc70/Hsp70 ATPase ...
(Bcl-2-associated athanogene 3), which transfers misfolded protein substrates bound to
HSP70
The 70 kilodalton heat shock proteins (Hsp70s or DnaK) are a family of conserved ubiquitously expressed heat shock proteins. Proteins with similar structure exist in virtually all living organisms. Intracellularly localized Hsp70s are an importa ...
(heat-shock protein 70) directly on to the microtubule motor dynein. The protein aggregate is then transported along the microtubule and unloaded via ATPase
p97 forming the aggresome.
Mediator
Mediator may refer to:
*A person who engages in mediation
*Business mediator, a mediator in business
* Vanishing mediator, a philosophical concept
* Mediator variable, in statistics
Chemistry and biology
*Mediator (coactivator), a multiprotein ...
s such as p62 are believed to be involved in aggresome formation in sequestering omega-somes, which bind and increase the size of the aggresome. The aggresome is eventually targeted for
autophagic
Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent re ...
clearance from the cell. Some pathological proteins, such as
alpha-synuclein
Alpha-synuclein 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.
It is abundant in the brain, while smaller a ...
, cannot be degraded and cause the aggresomes to form inclusion bodies (in
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
,
Lewy bodies
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 ( Parkinson's disease dementia and dementia with Lewy bodies (DLB)), and ...
) which contribute to neuronal dysfunction and death.
Triggering aggresome formation
Abnormal polypeptides that escape proteasome-dependent degradation and aggregate in cytosol can be transported via microtubules to an aggresome, a recently discovered organelle where aggregated proteins are stored or degraded by autophagy. Synphilin 1, a protein implicated in Parkinson disease, was used as a model to study mechanisms of aggresome formation. When expressed in naïve HEK293 cells, synphilin 1 forms multiple small highly mobile aggregates. However, proteasome or Hsp90 inhibition rapidly triggered their translocation into the aggresome, and surprisingly, this response was independent on the expression level of synphilin 1. Therefore, aggresome formation, but not aggregation of synphilin 1, represents a special cellular response to a failure of the proteasome/chaperone machinery. Importantly, translocation to aggresomes required a special aggresome-targeting signal within the sequence of synphilin 1, an ankyrin-like repeat domain. On the other hand, formation of multiple small aggregates required an entirely different segment within synphilin 1, indicating that aggregation and aggresome formation determinants can be separated genetically. Furthermore, substitution of the ankyrin-like repeat in synphilin 1 with an aggresome-targeting signal from huntingtin was sufficient for aggresome formation upon inhibition of the proteasome. Analogously, attachment of the ankyrin-like repeat to a huntingtin fragment lacking its aggresome-targeting signal promoted its transport to aggresomes. These findings indicate the existence of transferable signals that target aggregation-prone polypeptides to aggresomes.
Human disease
Accumulation of misfolded proteins in proteinaceous inclusions is common to many age-related
neurodegenerative
A neurodegenerative disease is caused by the progressive loss of structure or function of neurons, in the process known as neurodegeneration. Such neuronal damage may ultimately involve cell death. Neurodegenerative diseases include amyotrophic ...
diseases, including
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
,
Alzheimer's disease
Alzheimer's disease (AD) is a neurodegeneration, 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 short-term me ...
,
Huntington's disease
Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...
, and
amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND) or Lou Gehrig's disease, is a neurodegenerative disease that results in the progressive loss of motor neurons that control voluntary muscles. ALS is the most comm ...
. In cultured cells, when the production of misfolded proteins exceeds the capacity of the chaperone refolding system and the ubiquitin-proteasome degradation pathway, misfolded proteins are actively transported to a cytoplasmic juxtanuclear structure called an aggresome. Whether aggresomes are benevolent or noxious is unknown, but they are of particular interest because of the appearance of similar inclusions in protein deposition diseases. Evidence shows that aggresomes serve a cytoprotective function and are associated with accelerated turnover of mutant proteins. Experiments show that mutant androgen receptor (AR), the protein responsible for X-linked spinobulbar muscular atrophy, forms insoluble aggregates and is toxic to cultured cells. Mutant AR was also found to form aggresomes in a process distinct from aggregation. Molecular and pharmacological interventions were used to disrupt aggresome formation, revealing their cytoprotective function. Aggresome-forming proteins were found to have an accelerated rate of turnover, and this turnover was slowed by inhibition of aggresome formation. Finally, it is shown that aggresome-forming proteins become membrane-bound and associate with lysosomal structures. Together, these findings suggest that aggresomes are cytoprotective, serving as cytoplasmic recruitment centers to facilitate degradation of toxic proteins.
Proteins implicated in aggresome formation
Histone deacetylase 6 is the protein that, in the deacetylase adaptor protein function, forms Lewy bodies (the regular wild-type protein localized to
inclusion bodies
Inclusion bodies are aggregates of specific types of protein found in neurons, a number of tissue cells including red blood cells, bacteria, viruses, and plants. Inclusion bodies of aggregations of multiple proteins are also found in muscle cells ...
). No mutation associated with disease has been linked to this protein.
Parkin is the protein that, in the protein ligase function, forms Lewy bodies (the regular wild-type protein localized to inclusion bodies). Parkinson's disease has been linked to this protein when there is a protein.
Ataxin-3 is the protein that, in the deubiquitinating enzyme function, forms SCA type-1 and 2 DRPLA intranuclear inclusions (the regular wild-type protein localized to inclusion bodies). SCA type-3 has been linked to this protein when there is a protein.
Dynein motor complex is the protein that, in the retrograde
microtubule motor function, forms an unknown protein (the regular wild-type protein localized to inclusion bodies). Motor neuron degeneration has been linked to this protein when there is a protein.
Ubiquilin-1 is the protein that, in the protein turnover, intracellular trafficking function, forms Lewy bodies and neurofibrillary tangles (the regular wild-type protein localized to inclusion bodies). Alzheimer's disease (potential risk factor) has been linked to this protein when there is a protein.
Cystic fibrosis
Cystic fibrosis transmembrane conductance regulator
Cystic fibrosis transmembrane conductance regulator (CFTR) is a membrane protein and anion channel in vertebrates that is encoded by the ''CFTR'' gene.
Geneticist Lap-Chee Tsui and his team identified the CFTR gene in 1989 as the gene linked wit ...
(CFTR) is an inefficiently folded integral membrane protein that is degraded by the cytoplasmic ubiquitin-proteasome pathway. Overexpression or inhibition of proteasome activity in transfected
human embryonic kidney cell
Human embryonic kidney 293 cells, also often referred to as HEK 293, HEK-293, 293 cells, or less precisely as HEK cells, are a specific immortalised cell line derived from a spontaneously miscarried or aborted fetus or human embryonic kidney cells ...
s or
Chinese hamster ovary cell
Chinese hamster ovary (CHO) cells are an epithelial cell line derived from the ovary of the Chinese hamster, often used in biological and medical research and commercially in the production of recombinant therapeutic proteins. They have foun ...
s leads to the accumulation of stable, high molecular weight, detergent-insoluble, multi-ubiquitinated forms of CFTR. Undergraded CFTR molecules accumulate at a distinct pericentriolar aggresome.
Role of the aggresome pathway in cancer
There is emerging evidence that inhibiting the aggresome pathway leads to accumulation of misfolded proteins and
apoptosis
Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
in tumor cells through
autophagy
Autophagy (or autophagocytosis; from the Ancient Greek , , meaning "self-devouring" and , , meaning "hollow") is the natural, conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent re ...
.
See also
*
JUNQ and IPOD
JUNQ and IPOD are types of cytosolic protein inclusion bodies in eukaryotes.
Neurodegenerative diseases, such as Parkinson's, Alzheimer's, and Huntington's, are associated and correlated with protein aggregation and accumulation of misfolded ...
References
Further reading
Aggresomes: A Cellular Response to Misfolded ProteinsAggresomes protect cells by enhancing the degradation of toxic polyglutamine-containing protein*
Aggresome Formation and Neurodegenerative Diseases: Therapeutic ImplicationsRole of the Aggresome Pathway in Cancer: Targeting Histone Deacetylase 6–Dependent Protein Degradation* {{cite journal , pmid = 18635553 , doi=10.1074/jbc.M802216200 , volume=283 , issue=41 , title=Triggering aggresome formation. Dissecting aggresome-targeting and aggregation signals in synphilin 1. , date=Oct 2008 , journal=J Biol Chem , pages=27575–84 , last1 = Zaarur , first1 = N , last2 = Meriin , first2 = AB , last3 = Gabai , first3 = VL , last4 = Sherman , first4 = MY, doi-access = free
Cell biology