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Neurodegeneration is the umbrella term for the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases including Parkinson’s, Alzheimer’s, and Huntington’s occur as a result of neurodegenerative processes. As research progresses, many similarities appear which 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 models of worms (C. elegans), fruit flies (Drosophila), mice, and non-human primates. It is important to note that mammalian data is often needed for FDA approval of drugs, so a 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.
- 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.
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 structues: 
- 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 .
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 .
Programmed Cell Death
Programmed cell death (PCD) is death of a cell in any form, mediated by an intracellular program . 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) which result in the activation of caspases-8 or -10 .
- Intrinsic apoptotic pathways: result from mitochondrial release of cytochrome c or endoplasmic reticulum malfunctions both of which lead 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 which 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 .
PCD and Neurodegeneration
Current research, often in transgenic animal models, implicates both apoptotic and non-apoptotic pathways in neurodegeneration. Different diseases may enter these pathways at different points, but once triggered can lead to interdependent pathways of cell death . Generally, cell death in neurodegeneration is due to apoptosis and most commonly through the intrinsic mitochondrial pathway .
- Main article: Alzheimer's disease
The following paragraph was taken from the Alzheimer’s disease page.
Alzheimer's disease is characterised by loss of neurons and synapses in the cerebral cortex and certain subcortical regions. This loss results in gross atrophy of the affected regions, including degeneration in the temporal lobe and parietal lobe, and parts of the frontal cortex and cingulate gyrus.
Alzheimer's disease has been identified as a protein misfolding disease (proteopathy), caused by accumulation of abnormally folded A-beta and tau proteins in the brain. Plaques are made up of small peptides, 39–43 amino acids in length, called beta-amyloid (also written as A-beta or Aβ). Beta-amyloid is a fragment from a larger protein called amyloid precursor protein (APP), a transmembrane protein that penetrates through the neuron's membrane. APP is critical to neuron growth, survival and post-injury repair. In Alzheimer's disease, an unknown process causes APP to be divided into smaller fragments by enzymes through proteolysis. One of these fragments gives rise to fibrils of beta-amyloid, which form clumps that deposit outside neurons in dense formations known as senile plaques.
- Main article: Parkinson's disease
The following paragraph is an excerpt from the Pathophysiology section of the article Parkinson's disease. The mechanism by which the brain cells in Parkinson's are lost may consist of an abnormal accumulation of the protein alpha-synuclein bound to ubiquitin in the damaged cells. The alpha-synuclein-ubiquitin complex cannot be directed to the proteosome. This protein accumulation forms proteinaceous cytoplasmic inclusions called Lewy bodies. The latest research on pathogenesis of disease has shown that the death of dopaminergic neurons by alpha-synuclein is due to a defect in the machinery that transports proteins between two major cellular organelles — the endoplasmic reticulum (ER) and the Golgi apparatus. Certain proteins like Rab1 may reverse this defect caused by alpha-synuclein in animal models.
Recent research suggests that impaired axonal transport of alpha-synuclein leads to its accumulation in the Lewy bodies. Experiments have revealed reduced transport rates of both wild-type and two familial Parkinson’s disease-associated mutant alpha-synucleins through axons of cultured neurons.
- Main article: Huntington's disease
The following paragraph is an excerpt from the Mechanism section of the article Huntington's disease.
HD causes astrogliosis and loss of medium spiny neurons Areas of the brain are affected according to their structure and the types of neurons they contain, reducing in size as they cumulatively lose cells. The areas affected are mainly in the striatum, but also the frontal and temporal cortices. The striatum's subthalamic nuclei send control signals to the globus pallidus, which initiates and modulates motion. The weaker signals from subthalamic nuclei thus cause reduced initiation and modulation of movement, resulting in the characteristic movements of the disorder.
Mutant Huntingtin is an aggregate-prone protein. During the cells' natural clearance process, these proteins are retrogradely transported to the cell body for destruction by lysosomes. It is a possibility that these mutant protein aggregates damage the retrograde transport of important cargoes such as BDNF by damaging molecular motors as well as microtubules. .
Amyotrophic Lateral Sclerosis (ALS)
- Main article: Amyotrophic Lateral Sclerosis
Amyotrophic lateral sclerosis (ALS/Lou Gehrig’s Disease) is a disease in which motor neurons are selectively targeted for degeneration. In 1993, missense mutations in the gene encoding the antioxidant enzyme Cu/Zn superoxide dismutase 1 (SOD1) were discovered in subsets of patients with familial ALS. This discovery led researchers to focus on unlocking the mechanisms for SOD1-mediated diseases. Unfortunately, the pathogenic mechanism underlying SOD1 mutant toxicity has yet to be resolved.
Recent independent research by Nagai et al.  and Di Giorgio et al.  provide in vitro evidence that the primary protein mutation occurs in astrocytes. Astrocytes then cause the toxic effects on the motor neurons. The specific mechanism of toxicity still needs to be investigated, but the findings are significant because they implicate cells other than neuron cells in neurodegeneration .
Aging and Neurodegeneration
The greatest risk factor for neurodegenerative diseases is aging. Mitochondrial DNA mutations as well as oxidative stress both contribute to aging . Many of these diseases are late-onset, meaning there is some factor that changes as a person ages for each disease . One constant factor is that in each disease, neurons gradually lose function as the disease progresses with age.
Animal research offers an ideal solution to testing therapeutic strategies. Model organisms provide an inexpensive and relatively quick means to perform two main functions: target identification and target validation. Together, these help show the value of any specific therapeutic strategies and drugs when attempting to ameliorate disease severity. An exapmle is the drug Dimebon (Medivation). This drug is in phase III clinical trials for use in Alzheimer’s disease, and also recently finished phase II clinical trials for use in Huntington’s disease .
In another experiment using a rat model of Alzheimer's disease, it was demonstrated that systemic administration of hypothalamic proline-rich peptide (PRP)-1 offers neuroprotective effects and can prevent neurodegeneration in hippocampus amyloid-beta 25–35. This suggests that there could be therapeutic value to PRP-1.
Protein degradation offers therapeutic options both in preventing the synthesis and degradation of irregular proteins. There is also interest in upregulating autophagy to help clear protein aggregates implicated in neurodegeneration. Both of these options involve very complex pathways that we are only beginning to understand .
The goal of immunotherapy is to enhance aspects of the immune system. Both active and passive vaccinations have been proposed for Alzheimer's disease, however more research must be done to prove safety and efficacy in humans .
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