Discusses how the specific diseases, Huntington’s, ALS, prion diseases, Alzheimer’s, Parkinson’s and others relate to autophagy induction
Here are some details on the diseases we are trying to avoid and how they may fit the model of diseases that autophagy might benefit:
Huntington’s Disease (HD) is a rare neurodegenerative disease affecting at most around 7 people in 100,000 that produces a mental and physical decline starting most often in mid-life and ending in death. Woody Guthrie is its most famous victim. It is characterized by condensates of a protein named for the disease – huntingtin.
In many ways, HD is perhaps the best available research target for investigating neurodegenerative diseases generally. Its cause is entirely genetic and, in genetics parlance, is an autosomal dominant. This means that the disease results from the presence of a defective gene product rather than from the absence of a functional gene product as characterizes recessive or sex-linked genetic diseases. This makes tracing cause and effect morestraightforwardthan otherwise.
Every cell has the defect and so no mechanism to account for cell-to-cell transmission is required. (That does not mean there is in reality no transmission.)
The gene mutation that results in HD is known and gene testing for it is routinely available. This means that a population can be identified who will eventually develop symptoms of the disease long before it manifests. These people, because it is untreatable and so devastating and because they likely have already seen the devastation in their relatives, are often willing and eager volunteers for scientific studies.
Serving as guinea pigs may be the only medical attention they can get for their condition. Since they do not yet have disease symptoms, physicians and insurers are generally unwilling to get involved. Providing an alternative to this population that does not require professional medical involvement was indeed the inspiration for this book.
Specifically the disease is caused by an defect in a region of a specific protein, huntingtin, where a chain of the amino acid glutamine that is normally around 16 to 24 glutamines in length is expanded to a length exceeding 35. The expansion in the protein is from an expansion in the gene that encodes the protein: a normal 16 glutamine chain is encoded in the DNA by 16 repeats of the nucleotide triplet corresponding to glutamine, CAG. In HD, the CAG repeats are expanded to greater than 35 and so the disease is characterized as a ‘CAG expansion disease’, as are a number of other even rarer diseases. This expansion from the normal is transmitted through the generations in families that carry the disease.
Even though the disease is an autosomal dominant and generally proves eventually fatal, the gene has not been weeded out of the human genome. There are several explanations for this. Usually the disease manifests in the post-reproductive life of its sufferers after the gene has already been passed on. Also people with HD tend to have more children than average apparently because of some mental affect of HD that manifests earlier than the physical affects. This must compensate for the otherwise negative selection that normally extinguishes autosomal dominant genetic diseases.
It is unlikely that HD routinely arises from spontaneous mutation as do many other autosomal dominant diseases. It has been around for centuries at least and, though found in all populations, is far more prevalent in those of Western European descent. It would be evenly distributed if it were from a common frequent mutation event.
Occasionally HD will appear where there is no family history of the disease. Often this is from ignorance and denial of the true causes of decline and death within a family. Another reason is that the length of the CAG expansion tends occasionally to increase from generation to generation, sometimes dramatically. Thus a 36 CAG repeat gene that would normally not produce the disease can be expanded to a length that does. For some reason, this expansion, when it does occur, is more common when inherited from the father rather than the mother.
The ‘happy’ consequence of all this to researchers is that the HD gene comes in a whole range of variants differing in the length of the CAG expansion. The larger the expansion, the earlier, on average, the disease manifests though the character and progression of the disease is mostly unchanged. Animal models of the disease can thus be created with different expansions yet with some confidence that they mimic some real instance of the human disease.
Indeed a wide range of animal (and plant) models have been developed for HD including yeast, fruit flies, human and other mammalian cell lines, mice, and even sheep with any number of CAG repeats or portion of the huntingtin protein. In some the protein can be conditionally produced depending on say the presence or absence of a particular drug or hormone.
As mentioned, there are other CAG expansion diseases besides HD, though they are all much rarer. They currently include eight forms of spinocerebellar ataxias and a disease abbreviated as DRPLA. In each, the particular protein species with the CAG expansion is different and otherwise unrelated to the protein species of the others. Regardless, the diseases are all similar in that afflicted neurons progressively decline until a particular nerve tissue or type fails.
Scientists can even create new CAG expansion diseases; one team added a jelly-fish fluorescence protein gene with a CAG repeat to an animal model and observed consequent neurodegeneration50.
In all cases it is the CAG repeat protein with the expanded poly-glutamine sequence that forms aggregates. Clearing the aggregates by inducing autophagy could be effective on all forms.
CJD and the Prion Diseases
Creutzfeldt-Jakob disease (CJD) is the most common human instance of a class of neurodegenerative diseases called transmissible spongiform encephalopathies (TSEs). The class includes a disease of sheep, scrapie, and of cattle, bovine spongiform encephalopathy (BSE), which can be transmitted to humans and has so disrupted the international trade in beef of late.
It is extremely rare and nothing we would change our diet to avoid (apart from avoiding suspect beef). It is of interest here because it is transmissible and may be the model of how non-genetic neurodegenerative diseases like AD, ALS and PD can spread from one cell to another.
Unlike all other known diseases, the infectious agent of TSEs is a protein and not a nucleic acid, DNA or RNA. This protein, the prion, is a mis-folded version of a protein found in the infected host. In ways as we discussed in the chapter on protein folding, the mis-folded protein interacts with the normal and causes it to mis-fold and aggregate as well, disrupting and eventually killing the cell. What we have yet to discuss is how this mis-folding cascade within one cell could transmit to other cells.
The normal protein whose mis-folded form is the prion is named PrP. It is a common protein found in or on the outer surface of most (if not all) cells and may serve in a trans-membrane transport or signaling role. When mis-folded, it forms aggregates with itself that may eventually condense to form inclusions or plaques just like the other neurodegenerative diseases. And also like the others, there are inherited genetic forms that account for some fraction of the cases (10-15% for CJD).
Somehow the mis-folded PrP proteins from the doomed cell must make contact with the same proteins in neighboring cells. The likely channel is through a process discussed earlier, endocytosis . It is a way cells take in many nutrients and the only way they can renew the cell membrane. When a cell dies, its neighbors use endocytosis to dispose of the body51. Specialized motile cells called macrophages also participate. In so doing they all bring the dead neighbor’s proteins into vesicles within their cytoplasm. Normally these vesicles are merged with lysosomes exactly as in autophagy and the contents thoroughly digested for re-cycling. The mis-folded PrP protein, however, is resistant to the enzymes of the lysosome by virtue of its aggregation. When the lysosome itself decays or is destroyed as must eventually happen, the mis-folded PrP is released into the heart of its next victim.
This pattern accounts for the ‘spongiform’ (sponge-like) character of these diseases. A large hole appears in the afflicted tissue as the prion spreads from cell to neighboring cell, killing as it goes. The infected motile macrophages may travel to other areas of the tissue to start new holes when they die. Or the prion released from the burst dead cell may travel on its own to remote sites. Either way a slew of holes now develops in the afflicted tissue and it begins to take on the appearance of a sponge.
ALS and other Motor Neuron Diseases
Amyotrophic Lateral Sclerosis (ALS) is the most common human instance of a class of neurodegenerative diseases called motor neuron diseases (MND). Its most famous victim is Lou Gehrig and is about as common (or rare) as Woody Guthrie’s HD. It is often characterized by condensates of neurofilament proteins and mutations in genes for a protein called superoxide dismutase (SOD). For unsurprising reasons, the motor neurons, by far the longest cells in the body, are least able to handle neurofilament malfunction and are the first cell type to die.
It is a progressive disease and, though it often first appears on only one side of the body, it eventually appears on the other as well.
More than for AD or PD, ALS is felt by many to arise from some specific, potentially identifiable cause such as exposure to pesticides and other toxins or even mechanical injury. Clusters of cases have been observed among unrelated people leading to this view, but the presence of inheritable forms, known (5-10% of cases) and unknown, currently clouds the issue.
If it is truly not genetic, then there must be some way for the injured cell to transmit its injury to other cells or the disease would not be progressive. Perhaps the endocytosis mechanism proposed for the prion diseases applies here as well.
It should be noted that cell-to-cell transmission does not require that the cells be of the same type or that they all suffer the consequences of ‘infection’ equally. The motor neuron could simply be the cell type least able to manage the aggregate and the type for which it first proves lethal.
The benefits of autophagy to a mouse model of ALS have recently been directly demonstrated52. An autophagy promoter, lithium, increased survivability whereas an autophagy inhibitor, 3-MA, decreased it. Autophagy was directly observed in the cells and cited as the reason for the benefit.
Unlike ALS that can strike people of any age, Alzheimer’s Disease (AD) shows a strong preference for the aged and its study is complicated by all the other diseases that commonly accompany old age. Nevertheless, as we see in the mortality charts, it is the most common of the class of diseases called dementias and of all neurodegenerative diseases generally.
This is the disease we most want to avoid. The mean life expectancy following diagnosis is approximately seven years53, marked by dependency and decline ending in death (if some other disease does not get there first). Fortunately it fits the pattern of diseases that might be prevented by pre-symptomatic autophagy induction by protein cycling or ADCR: it is progressive, there is an associated cellular protein condensate, and some evidence suggests that autophagy benefits the condition.
Again it is characterized by condensates of a protein called ‘tau’ and fragments of a protein called ‘amyloid precursor protein’. Unlike the other diseases discussed here, condensates appear outside the cell as well as inside. There is some evidence that lysosomes may exocytose when they have fully digested their contents and thereby dump any undigestibles outside the cell. This might explain how the aggregate proteins in AD appear as plaques outside the cell as well as within54. It may also explain how aggregate proteins could move to other cells to seed new mis-folding cascades.
And, like ALS, lithium, another autophagy inducer shows benefits as well57– including condensate clearance.
Parkinson’s Disease and Other Lewy Body Diseases
Parkinson’s Disease (PD) is the second most common neurodegenerative disease after AD. Inherited forms are very rare. Though progressive in its symptoms, it is rarely terminal of itself.
The disease appears as a particular tissue in the brain, the substantia nigra, deteriorates. The substantia nigra normally produces the substance, dopamine, necessary to the cells that direct muscle movement. Any condition that destroys this tissue will produce parkinsonism, the paralysis characteristic of the disease. Parkinson’s Disease, however, is a specific progressive condition for which parkinsonism is only one aspect. Among the other aspects is dementia which may occur without parkinsonism. One aspect common to all forms of the disease is the presence of condensates called Lewy bodies so, in the case of dementia in the absence of parkinsonism, the disease may instead be labeled as dementia with Lewy bodies (DLB).If the Lewy bodies are in the neuron support cells called glia cells instead of in the neurons themselves, the disease is labeled as multiple system atrophy (MSA)
Regardless these diseases also fit the pattern of those that might be prevented by presymptomatic autophagy induction by protein cycling or ADCR: it is progressive, there is an associated cellular protein condensate, and some evidence suggests that autophagy benefits the condition.
It is characterized by condensates (the Lewy bodies) of a protein of unknown function called ‘alpha-synuclein’. Interestingly fragments of this protein appear in some AD plaques as well.
And like the other neurodegenerative diseases, the autophagy promoters rapamycin58, curcumin59, and lithium60clear the aggregate and/or inhibit disease progression. Also some genetic forms of PD have been traced to a lack of a protein needed to autophage damaged mitochondria61.
There is substantial evidence that PD can sometimes result from head injuries62even though the disease manifests long after the trauma and then gets progressively worse. The protein mis-folding model might explain this rather puzzling observation. Perhaps the injury temporarily disrupts the blood-brain-barrier and allows circulating mis-folded alpha-synuclein fragments, harmless elsewhere, into the brain where they then seed a mis-folding cascade.
Other Degenerative Diseases
All the diseases discussed here are classed as ‘proteopathies’63as a specific protein species is seen to misbehave. There are other tissues besides neurons with progressive degenerative diseases characterized by cellular inclusions, most notably muscle. The list is long,including some not uncommon diseases like amyloidosis, but most arerare and I will not go into them. Nevertheless, where protein aggregates are involved, autophagy as induced by protein cycling or ADCR might be preventative for them as well.
Speaking of amyloidosis, there is evidence that type 2 diabetes (adult onset) may be caused by amyloid plaque formation in pancreatic cells64. A protein called amylin forms the aggregate. Perhaps even diabetes may be prevented or delayed by protein cycling. (Or is it just that everything looks like a nail to a man with a hammer?)
There are certainly progressive degenerative diseases that do not involve aggregates. The auto-immune diseases like multiple sclerosis, rheumatoid arthritis, diabetes, etc. come to mind. The affect of protein cycling on their development is unknown but for them I have no plausible scientific rationale to believe promoting autophagy would be positive, neutral or negative.
There is, however some evidence that protein cycling might benefit these diseases (and others such as scleroderma, inflammatory bowel disease, eczema and psoriasis) by a mechanism not yet tied directly to autophagy. The drug halofuginone, derived from hydrangeas, is effective against autoimmune inflammation. A recent study65showed that it works by stimulating the cell’s amino acid starvation response, and, of course, protein cyclingisamino acid starvation.