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Tag: neurodegeneration

Breaking Research: A new technique for studying axon death using fruit fly wings

labeled neuronThe axon is the part of a neuron that carries outgoing information. (cb = cell body)

In neurodegenerative diseases such as Parkinson’s disease or amyotrophic lateral sclerosis (ALS), a genetic mutation leads to widespread neuron damage. When a neuron is damaged, its axon—the part of the neuron that carries outgoing signals—is actively broken down and cleared away by maintenance cells (called glia). This destructive process is known as Wallerian degeneration (WD) and disrupts signaling between neurons due to the loss of their axons. By studying WD in animal models, scientists hope to figure out how to interfere with it and possibly slow the progression of the neurodegenerative diseases.

Because Wallerian degeneration also occurs after physical trauma such as in a spinal cord injury, scientists usually study it by cutting or crushing bundles of axons. Traditionally, researchers have induced random genetic mutations in a population of animals (a forward genetic screen), dissected and damaged a bundle of axons in individual animals, and watched the degradation process to see if the mutation had an effect. This process allows them to determine which genes are involved in breaking down and/or clearing away the damaged axons. So far, studies in mammals and fruit flies have found three genes that are required for WD (Wld, Sarm, and highwire/Phr1). Mutating these genes slows WD significantly, and researchers may be able to target these genes in the future to slow axon death.

But progress so far has been slow due to limitations in the current methods used for studying this phenomenon. In a recent paper published in PNAS by the Freeman lab, researchers describe a new technique for studying Wallerian degeneration in fruit flies that overcomes many of these limitations. Previous work has already shown that WD occurs in fruit fly axons and uses the same known genes, so they are an ideal model for quickly identifying the genes involved in axon death.

The authors developed a system in the fly wing that fulfilled three key requirements they believed were necessary for efficiency:

  1. Rapid observation of individual axons (rather than an entire axon bundle) with minimal dissection
  2. The opportunity to sever only a subset of axons so that one could observe axon death alongside healthy, uninjured axons
  3. The ability to initiate and visualize axon death without killing the fly
fly wingTop: A fly wing with sensory neurons labeled in green. (cb = cell body). Inset: a bundle of labeled axons makes up the L1 vein. Bottom: A schematic of the fly wing and cutting a subset of axons (dashed line). Axons with cell bodies closer to the fly’s body are spared.

The Drosophila wing is transparent and contains a large bundle of sensory neuron axons known as the L1 vein. The cell bodies of the neurons are located around the edge of the wing, and their axons extend through the L1 vein back into the body of the fly. Using genetic tools (check out this article on MARCM for details), the authors added a protein called GFP (which glows green) in a small number of these sensory neurons. This setup allows researchers to easily observe individual axons in the wing under a fluorescent microscope without dissection. These characteristics fulfill their first requirement.

This setup also easily allows researchers to sever some of the axons without injuring others by simply snipping a piece from the edge of the wing. Axons whose cell bodies were cut off succumb to WD, while axons with cell bodies were closer to the fly’s body are unharmed. This method of inducing WD in only some of the GFP-labeled neurons fulfills the second requirement.

GFP axon being degradedThree GFP-labeled axons: two healthy, one damaged and undergoing Wallerian degeneration (white arrow).

Finally, because the entire procedure can be performed in the fly’s wing, the fly itself is generally unharmed. This fulfills the final requirement, which is important for speeding up the screening process. Usually, after random mutations are induced in a population of flies, a fly line for each mutation has to be maintained until it can be tested. But if the original fly can live through the testing process, researchers can first determine if fly’s mutation is useful and then allow the fly to mate and create a stock if it is. This characteristic not only reduces the time needed for the screen by weeks, but it also significantly reduces the amount of labor required.

So how can this help us humans? This technique can speed up the process for discovering new genes involved in Wallerian degeneration. After a gene is identified in flies, researchers will know where to look in mammals and can begin studying the gene’s function (this has already happened once, with the Sarm gene). Figuring out how these genes function will provide a target for slowing axon death, which may in turn slow the progression of neurodegenerative diseases. And it all starts with a fruit fly’s wing.

 
Reference:

  • Neukomm L.J., Burdett, T.C., M. A. Gonzalez, S. Zuchner & M. R. Freeman (2014). Rapid in vivo forward genetic approach for identifying axon death genes in Drosophila, Proceedings of the National Academy of Sciences, 111 (27) 9965-9970. DOI: http://dx.doi.org/10.1073/pnas.1406230111

Breaking Research: Glycogen build-up in the brain contributes to aging

young vs old brainTotal brain volume decreases as we age. Image modified from brainpowerrelease.

Why is the aging process accompanied by progressive cognitive decline such as impaired memory, decreased focus, and slowed reaction time? Although we don’t fully know what causes it, researchers have found that aging visibly affects the brain, most strikingly as a decrease in total brain volume due to progressive loss of neurons. As we age, it is thought that neurons are irreversibly damaged by cumulative mistakes in building proteins, copying DNA, and other processes.

Sometimes, these mistakes can cause abnormal proteins and other molecules to clump together and form dense deposits inside neurons. In many neurodegenerative diseases such as Alzheimer’s or Lafora’s disease, an excess of deposits is associated with rapid neuron death. Yet although the (albeit much less severe) accumulation of deposits in the brain is a normal part of aging, its role in cognitive decline has been historically ignored. In a new study published in Aging Cell by the Milán and Guinovart labs, researchers investigate why these deposits appear during the normal aging process and what effect they have on neuronal function.

glycogen moleculeGlycogen is a storage molecule made up of thousands of branching glucose units. Image by Mikael Häggström.

Previous research has shown that the deposits present in aged human brains (which are referred to as either corpora amylacea or polyglucosan bodies; I’m just going to call them PGBs) are primarily made up of glycogen molecules, along with some other proteins. Glycogen is an important energy storage molecule in animal cells (second only to fats) and each glycogen molecule is composed of thousands of glucose units. But what causes the glycogen to aggregate and form deposits in a normal aging brain?

PGBs in mouse brainTop: brain (hippocampus) slice of aged normal mouse, stained with PAS, scale bar 200um. Bottom: Zoomed in on black box, arrow points to PGBs. Modified from Sinadinos et al, 2014.

The authors analyzed PGBs in the brains of aged mice to answer this question. They first studied the composition of the mouse PGBs and confirmed that they were similar to the PGBs found in aged human brains, down to the types of proteins bound up among the glycogen molecules. Next, they created a knock-out mouse line that was missing the glycogen synthase gene, which synthesizes glycogen by attaching glucose molecules together. The authors couldn’t find PGBs in the aged brains of these mutant mice, thus demonstrating that the process of synthesizing glycogen contributed to PGB formation. This finding is admittedly unsurprising: if the cells can’t make glycogen, it’s not there to clump together.

Well here’s the interesting part! Not only couldn’t the authors find glycogen deposits, but they also didn’t see accumulation of any of other protein associated with PGBs. One of the proteins they tested was alpha-synuclein, the aggregate-prone protein involved in Parkinson’s disease. In the normal mice, alpha-synclein accumulated along with the PGBs, but there was no accumulation in the mutant mice. Thus, glycogen synthesis seemed to be a prerequisite for the formation of other protein deposits. This finding could have implications for possible treatments to slow aging—if we can interfere with glycogen synthesis, could we stop the accumulation of other damage-causing proteins and reduce the detrimental effects on the brain?

To answer that question, we’d need to first confirm that the PGBs are actually involved in the decline of neuronal function during aging. This leads me to the authors’ next set of experiments, for which they turned to Drosophila melanogaster. Because fruit flies have shorter lifespans, it’s easier to study how PGBs affect them over their entire lives. In addition, fly researchers have developed a vast array of genetic tools for their animal model, which the authors used to knock-out the glycogen synthase gene again, but this time only in the brain and only during adulthood. This allowed the researchers to study how PGB formation affects brain function without altering the development and general health of the fly (a very difficult feat in mice).

As expected, the authors found that the mutant flies had reduced levels of glycogen in the brain. But remarkably, they also found that the mutants lived significantly longer than the normal flies. And this was quality life—the aged mutant flies could climb better and faster than normal flies of the same age. Based on these results, the authors concluded that glycogen synthesis impairs neuronal function and survival with age.

The findings from this paper is a step in the right direction for figuring out why cognitive decline is associated with aging. Before this can be useful for humans, however, there are a lot of questions to answer. How and why does glycogen synthesis cause PGB formation as we age? Will interfering with this process in adulthood extend quality life and cognitive function (and can it be done safely)?  Finally, the finding that glycogen synthesis may be required for other protein deposits was completely unexpected. Future work in this field may therefore provide insights into neurodegenerative diseases associated with protein aggregation, such as Alzheimer’s disease (beta-amyloid), Parkinson’s disease (alpha-synuclein), and Lafora’s disease (glycogen!).
 
 
For more research on aging, click the “aging” tag in the right column.

For another post about how to reduce aging-related protein aggregation, check out this post about the possible use of lithium.

 

 
Reference:

  • Sinadinos C., Laura Boulan, Estel Solsona, Maria F. Tevy, Mercedes Marquez, Jordi Duran, Carmen Lopez-Iglesias, Joaquim Calbó, Ester Blasco, Marti Pumarola, Marco Milán & Joan J. Guinovart (2014). Neuronal glycogen synthesis contributes to physiological aging, Aging Cell, n/a-n/a. DOI: http://dx.doi.org/10.1111/acel.12254

General References:

Breaking Research: Lithium may protect against Alzheimer’s and other aging-related diseases

As human life expectancy continues to increase at a steady rate in most countries worldwide, the prevalence of aging-related diseases is also increasing. One such example is Alzheimer’s disease (AD), the most common cause of dementia in the aging population. There is currently no cure for AD, and the only treatments that exist temporarily cover up the symptoms without actually slowing the disease itself.

Alzheimer’s disease can be identified by the abnormal accumulation of a protein called amyloid beta (Aβ) in the brain. Aβ is a byproduct of an important cellular process, and is usually cleared away by the cell’s garbage recycling processes. It’s normal for some leftovers to be missed, however, and over time Aβ builds up as we age. But in large enough concentrations—such as in older patients with AD—accumulation leads to the formation of aggregated deposits of Aβ. These deposits damage neurons and cause neurodegeneration (progressive neuron death).

healthy vs Alzheimer's neuronsThe left side shows healthy neurons. The right side shows damaged neurons and Aβ deposits as seen in Alzheimer’s disease patients. Image modified from alz.org

Eventually, this widespread brain tissue damage leads to imbalances in the levels of neurotransmitters, which are chemicals that neurons use to communicate. This imbalance is thought to cause the symptoms of AD, including problems with memory, thinking, or changes in behavior. Current treatments simply alter the amount of these neurotransmitters without doing anything to slow or prevent Aβ aggregation and neuron death. Therefore, research aimed at developing a drug that can interfere with Aβ accumulation is important for treating this disease.

Recent findings in lithium research holds hope for such a treatment. Although lithium is most widely used as a treatment for bipolar disorder, some preliminary research suggests that it might also be able to slow or prevent the symptoms of Alzheimer’s disease in humans if prescribed early enough. However, the results have been mixed and even sometimes contradictory. This is largely due to the fact that lithium’s actions in the brain are not understood, so figuring out how lithium might be helping AD patients is essential before it could be considered as a treatment. A recent paper published in Frontiers of Aging Neuroscience by the Partridge lab has begun to do just that by studying how lithium can reduce Aβ accumulation in a fruit fly model of Alzheimer’s disease.

The authors created a model for AD by introducing a mutated form of human Aβ protein known to cause AD in some families (called Arctic Aβ42) into adult fruit flies. Flies with the Arctic Aβ42 mutation displayed progressive neuron dysfunction and shortened lifespan, which mimicked the symptoms of AD in human patients. They had previously shown that lithium treatment was able to reduce the amount of Aβ (and thus also its toxic effects on neurons) in these flies. But how did it work?  In this study, they treated these mutant flies with lithium again to answer this question.

What they found was surprising. Lithium didn’t just reduce the amount of Aβ protein in the brain, it reduced the amount of all proteins.  It worked by suppressing the activity of the “translation machinery”, which refers to the system that actually assembles and produces proteins from the instructions in the genetic code. So lithium actually reduced the production of all proteins through a mechanism that wasn’t specific to Aβ.

What does this mean for the possibility of using lithium to treat Alzheimer’s disease? The fact that lithium’s effects are more general is actually pretty good news not just for AD, but for all research into the aging process. It is currently thought that normal aging is caused by the accumulation of damaged or mutated proteins that haven’t been cleaned up, and research aimed at increasing lifespan has focused on either improving the cell’s recycling processes or reducing protein production. In fact, lithium treatment has been shown to increase life expectancy in animal models, including fruit flies (and possibly even in humans!).

So if lithium can increase lifespan in animals without Alzheimer’s disease, can it reverse the lifespan reduction in the AD model flies? Yes! The authors found that lithium treatment also improved the life expectancy of their mutant flies compared to ones that did not get a lithium treatment. This result provides further hope that lithium could one day be used to actually slow the progression of AD and give patients more years of quality life.

The findings in this paper are not just promising for Alzheimer’s disease, but also for other aging-related diseases caused by abnormal accumulation of protein in the brain, such as Parkinson’s disease and Huntington’s disease. Lithium also has the advantage of already being an approved drug for treating patients with bipolar disease, so some information on side-effects and dosages already exists. Of course, this doesn’t mean doctors should begin prescribing lithium for AD patients right away; the dosage requirements will likely be different and older adults may experience other side-effects. But research in this field has definitely leapt forward, and we may see a cure for aging-related diseases in our (extended?) lifetime.

Reference:

  • Sofola-Adesakin O., Jorge I. Castillo-Quan, Charalampos Rallis, Luke S. Tain, Ivana Bjedov, Iain Rogers, Li Li, Pedro Martinez, Mobina Khericha, Melissa Cabecinha, Jürg Bähler & Linda Partridge(2014). Lithium suppresses Aβ pathology by inhibiting translation in an adult Drosophila model of Alzheimer’s disease, Frontiers in Aging Neuroscience, 6 DOI: http://dx.doi.org/10.3389/fnagi.2014.00190

General References:

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