Neurodegeneration is the umbrella term for the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases including ALS, Parkinson’s, Alzheimer’s, and Huntington’s occur as a result of neurodegenerative processes. Such diseases are incurable, resulting in progressive degeneration and/or death of neuron cells. As research progresses, many similarities appear that relate these diseases to one another on a sub-cellular level.
Discovering these similarities offers hope for therapeutic advances that could ameliorate many diseases simultaneously. There are many parallels between different neurodegenerative disorders including atypical protein assemblies as well as induced cell death. Neurodegeneration can be found in many different levels of neuronal circuitry ranging from molecular to systemic.
Links Between Neurodegenerative Disorders
Many neurodegenerative diseases are caused by genetic mutations, most of which are located in completely unrelated genes. In many of the different diseases, the mutated gene has a common feature: a repeat of the CAG nucleotide triplet. CAG encodes for the amino acid glutamine. A repeat of CAG results in a polyglutamine (polyQ) tract. Diseases showing this are known as polyglutamine diseases.
Polyglutamine: A repeat in this causes dominant pathogenesis. Extra glutamine residues can acquire toxic properties through a variety of ways, including irregular protein folding and degradation pathways, altered subcellular localization, and abnormal interactions with other cellular proteins.
PolyQ studies often use a variety of animal models because there is such a clearly defined trigger – repeat expansion. Extensive research has been done using the models of nematode (C. elegans), and fruit fly (Drosophila), mice, and non-human primates. Mammalian data is often needed for FDA approval of drugs, which means that the bulk of the research is done using mice. Using data from the other animals (C. elegans and Drosophila primarily) is often a precursor to finding the equivalent mammalian gene.
Nine inherited neurodegenerative diseases are caused by the expansion of the CAG trinucleotide and polyQ tract. Two examples are Huntington’s disease and spinocerebellar ataxias. For a complete list see the table under Polyglutamine (PolyQ) Diseases in the article Trinucleotide repeat disorder. While polyglutamine-repeat diseases encompass many different neurodegenerative disorders, there are many more it does not apply to. The genetics behind each disease are different and often unknown.
Several neurodegenerative diseases are classified as proteopathies as they are associated with the aggregation of misfolded proteins.
- alpha-synuclein: can aggregate to form insoluble fibrils in pathological conditions characterized by Lewy bodies, such as Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy. Alpha-synuclein is the primary structural component of Lewy body fibrils. In addition, an alpha-synuclein fragment, known as the non-Abeta component (NAC), is found in amyloid plaques in Alzheimer’s disease.
- tau: hyperphosphorylated tau protein is the main component of neurofibrillary tangles in Alzheimer’s disease.
- beta amyloid: the major component of senile plaques in Alzheimer’s disease.
Protein Degradation Pathways
Parkinson’s disease and Huntington’s disease are both late-onset and associated with the accumulation of intracellular toxic proteins. Diseases caused by the aggregation of proteins are known as proteinopathies, and they are primarily caused by aggregates in the following structures:
- cytosol, e.g. Parkinson’s & Huntington’s
- nucleus, e.g. Spinocerebellar ataxia type 1 (SCA1)
- endoplasmic reticulum (ER), (as seen with neuroserpin mutations that cause familial encephalopathy with neuroserpin inclusion bodies)
- extracellularly excreted proteins, amyloid-β in Alzheimer’s disease
There are two main avenues eukaryotic cells use to remove troublesome proteins or organelles:
- ubiquitin–proteasome: protein ubiquitin along with enzymes is key for the degradation of many proteins that cause proteinopathies including polyQ expansions and alpha-synucleins. Research indicates proteasome enzymes may not be able to correctly cleave these irregular proteins, which could possibly result in a more toxic species. This is the primary route cells use to degrade proteins.
Decreased proteasome activity is consistent with models in which intracellular protein aggregates form. It is still unknown whether or not these aggregates are a cause or a result of neurodegeneration.
- autophagy–lysosome pathways: a form of programmed cell death (PCD), this becomes the favorable route when a protein is aggregate-prone meaning it is a poor proteasome substrate. This can be split into two forms of autophagy: macroautophagy and chaperone-mediated autophagy (CMA).
Macroautophagy is involved with nutrient recycling of macromolecules under conditions of starvation, certain apoptotic pathways, and if absent, leads to the formation of ubiquinated inclusions. Experiments in mice with neuronally confined macroautophagy-gene knockouts develop intraneuronal aggregates leading to neurodegeneration.
Chaperone-mediated autophagy defects may also lead to neurodegeneration. Research has shown that mutant proteins bind to the CMA-pathway receptors on lysosomal membrane and in doing so block their own degradation as well as the degradation of other substrates.
Damage to the membranes of organelles by monomeric or oligomeric proteins could also contribute to these diseases. Alpha-synuclein can damage membranes by inducing membrane curvature, and extensive tubulation and vesiculation were observed when these proteins were incubated with artificial phospholipid vesicles.
The most common form of cell death in neurodegeneration is through the intrinsic mitochondrial apoptotic pathway. This pathway controls the activation of caspase-9 by regulating the release of cytochrome c from the mitochondrial intermembrane space (IMS). Reactive oxygen species (ROS) are normal byproducts of mitochondrial respiratory chain activity. ROS concentration is mediated by mitochondrial antioxidants such as manganese superoxide dismutase (SOD2) and glutathione peroxidase.
Over production of ROS (oxidative stress) is a central feature of all neurodegenerative disorders. In addition to the generation of ROS, mitochondria are also involved with life-sustaining functions including calcium homeostasis, PCD, mitochondrial fission and fusion, lipid concentration of the mitochondrial membranes, and the mitochondrial permeability transition. Mitochondrial disease leading to neurodegeneration is likely, at least on some level, to involve all of these functions.
There is strong evidence that mitochondrial dysfunction and oxidative stress play a causal role in neurodegenerative disease pathogenesis, including in four of the more well known diseases Alzheimer’s, Parkinson’s, Huntington’s, and Amyotrophic lateral sclerosis.
Axonal swelling and spheroids have been observed in many different neurodegenerative diseases. This suggests that defective axons are not only present in diseased neurons, but also that they may cause certain pathological insult due to accumulation of organelles.
Axonal transport can be disrupted by a variety of mechanisms including damage to: kinesin and cytoplasmic dynein, microtubules, cargoes, and mitochondria. When axonal transport is severely disrupted a degenerative pathway known as Wallerian-like degeneration is often triggered.
Programmed Cell Death
Programmed cell death (PCD) is death of a cell in any form, mediated by an intracellular program. This process can be activated in neurodegenerative diseases including Parkinson’s disease, amytrophic lateral sclerosis, Alzheimer’s disease and Huntington’s disease. There are, however, situations in which these mediated pathways are artificially stimulated due to injury or disease.
Apoptosis (type I)
Apoptosis is a form of programmed cell death in multicellular organisms. It is one of the main types of programmed cell death (PCD) and involves a series of biochemical events leading to a characteristic cell morphology and death.
- Extrinsic apoptotic pathways: Occur when factors outside the cell activate cell surface death receptors (e.g., Fas) that result in the activation of caspases-8 or -10.
- Intrinsic apoptotic pathways: Result from mitochondrial release of cytochrome c or endoplasmic reticulum malfunctions, each leading to the activation of caspase-9. The nucleus and Golgi apparatus are other organelles that have damage sensors, which can lead the cells down apoptotic pathways.
- Caspases (cysteine-aspartic acid proteases) cleave at very specific amino acid residues. There are two types of caspases: initiators and effectors. Initiator caspases cleave inactive forms of effector caspases. This activates the effectors that in turn cleave other proteins resulting in apoptotic initiation.
Autophagic (type II)
Autophagy is essentially a form of intracellular phagocytosis in which a cell actively consumes damaged organelles or misfolded proteins by encapsulating them into an autophagosome, which fuses with a lysosome to destroy the contents of the autophagosome. Many neurodegenerative diseases show unusual protein aggregates. This could potentially be a result of underlying autophagic defect common to multiple neurodegenerative diseases. It is important to note that this is a hypothesis, and more research must be done.
Cytoplasmic (type III)
The final and least understood PCD mechanism is through non-apoptotic processes. These fall under Type III, or cytoplasmic cell death. Many other forms of PCD are observed but not fully understood or accepted by the scientific community. For example, PCD might be caused by trophotoxicity, or hyperactivation of trophic factor receptors. In addition to this, other cytotoxins that induce PCD at low concentrations act to cause necrosis, or aponecrosis – the combination of apoptosis and necrosis, when in higher concentrations. It is still unclear exactly what combination of apoptosis, non-apoptosis, and necrosis causes different kinds of aponecrosis.
Top Illustration: Jef Safi/Flickr