Neurodegeneration can occur more quickly if nerve cells lack retromer proteins, which are responsible for breaking down toxic tau protein. This conclusion was reached by scientists who observed genetically modified fruit flies that produced human tau protein in 39 neurons of the eye. Tau protein itself already caused the death of nerve cells, and the deficiency of retromers only exacerbated it – which may also happen in people when Alzheimer's disease develops. The study was published in the journal Nature Communications.
Many neurodegenerative disorders are caused by a toxic form of proteins. They accumulate in neurons and between them in the form of insoluble balls and disrupt the transport of substances and signal transmission. For example, in Parkinson's and Alzheimer's diseases, tau protein accumulates in tangles. Normally, it is part of the neuron cytoskeleton, but when excessively phosphorylated, it becomes toxic to cells.
Mutations in the gene encoding tau protein are associated with an increased risk of developing neurodegenerative diseases. However, the accumulation of aggregates can be caused not only by the breakdown of the protein itself, but also by disruption of the pathways that are responsible for the breakdown and degradation of protein coils. The retromer complex is one such pathway. In healthy cells, it is responsible for transporting protein molecules and receptors through endosomes to the Golgi apparatus network and back to the cell surface. Mutations in the proteins of this complex are associated with Parkinson's disease, and defects in its activity are responsible for the development of Alzheimer's disease. But until now, it was unclear exactly how these mutations are involved in the development of neurodegeneration.
A team of scientists from the Ecole Polytechnique Fédérale de Lausanne, led by Brian D. McCabe, tried to answer this question by reproducing the human mechanism of neurodegeneration in fruit flies. First, the authors of the work created a line of flies that expressed human tau protein as adults. The life expectancy of control and experimental individuals turned out to be different. Expression of tau protein reduced the lifespan of transgenic fruit flies by 40.5 percent (p < 0.001) relative to the control line of insects.
To understand what changes in nervous tissue led to high mortality, scientists created another transgenic insect model. It is known that high levels of tau protein lead to axonal degeneration, but it is difficult to observe this process experimentally due to the fact that many neurons in the brain produce the protein at once. Therefore, the researchers triggered expression in only one region of the Drosophila eye—the dorsal cluster (DC), which has only 39 neurons.
In healthy fruit flies, the number of synapses on DC neurons did not change within 25 days after hatching. Then there were fewer synapses, and by 40 days of life there were on average 18 percent fewer of them (p < 0.001) than in young individuals. In insects expressing human tau protein, the number of synapses remained unchanged until day 25, but by day 30 their number had decreased by 49 percent (p < 0.001), by 35 days by 75 percent (p < 0.001), and by 40 day - by 89 percent (p < 0.001) compared with control animals of the same age. The scientists concluded that the expression of tau protein in adult neurons leads to the progressive loss of synaptic connections. Additionally, by day 30, tau-expressing neurons had a 20 percent reduction in axon number (p < 0.001).
Then the researchers tested how the work of the retromer complex affects neurodegeneration. They blocked the production of individual proteins (Vps proteins) using RNA interference – and noticed that disturbances in eye development were even more pronounced than in insects with the expression of tau protein, by 20-40 percent. The authors also found that after blocking the retromer complex in flies, life expectancy is reduced and the number of axons in neurons is reduced.
Finally, the researchers asked why inhibiting retromer makes tau more toxic. They measured the level of phosphorylation, which is responsible for the toxicity of tau protein, and found that the work of the retromer complex does not affect it. However, due to the activity of caspase enzymes, shortened forms of the protein accumulated in the cell, which are considered more toxic. Scientists traced their localization in the cell and noticed that short forms of tau accumulate in late endosomes – intracellular vesicles that move proteins unnecessary for the cell to the lysosome, where they will be destroyed. The researchers suggested that the accumulation occurs precisely because of the absence of retromers, since they showed that these proteins interact with endosome proteins and help its function.
As a result, scientists proposed their own model of tau protein toxicity. When retromers are disrupted, the movement of toxic tau protein to the lysosome is greatly delayed. The slowdown occurs due to its accumulation in endosomes. At the same time, caspases, which are located in endosomal vesicles, act on tau protein longer and convert it into a shorter and more toxic one. An excess of this form of tau protein disrupts the stability of neurons and ultimately leads to the death of the animal.
Thus, the authors of the work have moved one step further in trying to reconstruct the full mechanism of neurodegeneration. They hope that their Drosophila model will help reveal more details about how and why neurons die, and that further research in this area will help determine human susceptibility to neurodegenerative diseases.
We have already written about previous discoveries in the field of the mechanism of neurodegeneration. We talked about how neuroscientists linked the accumulation of beta-amyloid with cholesterol levels, and also tested a vaccine against tau protein.