Posts tagged phosphorylation
Articles and news from the latest research reports.
Not guility: Parkinson and protein phosphorylation
EPFL scientists exonerated a process thought to play a role in causing Parkinson’s disease; rather than triggering toxic aggregates in neurons, it turns out that it actually slows down the disease, pharmas have now new tracks to explore
Clues left at the scene of the crime don’t always point to the guilty party, as EPFL researchers investigating Parkinson’s disease have discovered. It is generally accepted that the disease is aggravated when a specific protein is transformed by an enzyme. The EPFL neuroscientists were able to show that, on the contrary, this transformation tends to protect against the progression of the disease. This surprising conclusion could radically change therapeutic approaches that are currently being developed by pharmaceutical companies. The research is to appear in an article in the Proceedings of the National Academy of Sciences (PNAS).
Parkinson’s disease is characterized by the accumulation of a protein known as alpha-synuclein in the brain. If too much of it is produced or if it’s not eliminated properly, it then aggregates into small clumps inside the neurons, eventually killing them. Several years ago scientists discovered that these aggregated proteins in the brain had undergone a transformation known as “phosphorylation” — a process in which an enzyme adds an extra chemical element to a protein, thus modifying its properties.
The investigators’ conclusion that the enzyme’s activity could be responsible for the disease seems eminently reasonable. If phosphorylation and protein aggregation go hand in hand, then it makes sense that one should cause the other. This is the assumption that researchers and pharmaceutical companies made as they tried to reduce the phosphorylation by deactivating an enzyme involved in the process. But they have been following a false lead, as the EPFL team was able to show.
The scientists even discovered that the phosphorylation of the protein has positive effects. On the one hand, it considerably reduces the toxic aggregation of the protein, and on the other, it helps the cell eliminate the protein. “The two phenomena are undoubtedly related, and together could play a role in the reduction of alpha-synuclein toxicity, but we don’t yet understand the impact of both processes at each stage of the disease,” explains neurobiologist Abid Oueslati, first author on the study.
Going back to the beginning
To reach this conclusion, the biologists had to explore the initial disease conditions. They injected into rat neurons what were thought to be the elements needed to trigger the disease: an overexpression of alpha-synuclein and the enzyme that phosphorylates it (PLK2).
To their surprise, the group of animals subjected to both of the parameters — overproduction of the protein and phosphorylation — lost nearly 70% fewer neurons than another group in which only the protein was overexpressed. Consequently, they had fewer lesions, and less Parkinson symptoms.
"We owe this discovery to unique tools that we developed, in collaboration with the Aebischer group, in order to study the effect of this transformation at the molecular level. ," explains Hilal Lashuel, who directed the study. Our study revealed the limitations of the most commonly used approach, which uses genetic mutations to mimic this process.
Lashuel thinks it is highly probable that the phosphorylation of the proteins takes place after they are aggregated, that is to say once the disease is already established. Or it could be a defense mechanism of the neurons, an attempt to try and slow down the progression of the disease from the beginning.
The scientists’ research opens doors for the development of future drug therapies. “The lesson we learned from this research is that everything you find at the scene of a crime is not necessarily involved in the crime. By remaining fixated on that assumption, we may lose sight of the bigger picture.”
(Source: eurekalert.org)
Balancing mitochondrial dynamics in Alzheimer’s disease
Many diseases are multifactorial and can not be understood by simple molecular associations alone. Alzheimer’s disease (AD) is associated with toxic transformations in two classes of protein,amyloid beta and tau, but they do not explain the full underlying pathology. On the cellular scale, much of the real-time morphological changes in neurons can be attributed to their underlying mitochondrial dynamics—namely fission, fusion, and the motions between these events. Last year, researchers from Harvard Medical School made the intriguing discovery that alterations in tau could lead to a doubling in the length of mitochondria. This week, they published a review article in Trends in Neuroscience, in which they seek to explain the primary features of AD in terms of mitochondrial dynamics.
Together with a collaborator from the Queensland Brain Institute, the Harvard researchers arrive at the conclusion that, like many other neurological diseases, AD is fundamentally an energy problem. While some proteins, like APOE-ɛ4 can predispose one to AD, point defects in individual proteins can not account for AD in the same way that a single alteration in hemoglobin leads to sickle cell disease. Attempts to assign casual relations to the complex interactions of tau or amyloid, with hundreds of other proteins inside neurons have frequently served to cloud, rather than simplify the AD story.
In years gone by, it was possible to publish a paper about how phosphorylation at certain sites on proteins, like tau, could lead to any number of downstream events. Tau is one of many proteins that control the assembly and stability of microtubules, critical structures that are among those compromised in AD. The problem now, is that we know tau comes in so many flavors—it is a big family of different isoforms with different properties depending on how they are processed. As far as simple phosphorylation, tau has been found to have 79 potential sites, with at least 30 of them normally phosphorylated.
A welcome simplification to this situation of compounding molecular complexity, is that many pathways converge onto convenient pre-existing packets of time, space, and predictable molecular structure—the mitochondria. As opposed to massive cell-wide molecular accounting, describing a few sub-cellular morphological features may be a more tractable approach not only to capture disease etiology, but perhaps to treat it.
To this end, the researchers apply existing knowledge regarding some of the molecular players in AD, to a few of the well-established control points in mitochondrial dynamics. State transitions between fission and fusion are, at the moment at least, characterized by only a small handful of proteins. This simple formula might be prescribed as the following: molecular pathway locally effects the organelle dynamics, then, the dynamic behavior of organelle accounts for the disease. The imposition of this middleman can potentially simplify much of the vast body of fact and conjecture associated with the disease.
The elongation of mitochondria by tau can be caused by increasing fusion, decreasing fission, or both. One function of tau is to stabilize F-actin networks which prevents a key fission protein from ever reaching the mitochondria. Elongated mitochondria do not necessarily cause AD. In fact, amyloid beta, which is concentrated inside mitochondria, has been shown to cause increased fission and decreased fusion. When the balance between fission and fusion is pushed too far in either direction, the result is bad news for neurons. If there are defects in the transport of mitochondria, as seems to be the case in many neurological diseases, their redistribution is unable to compensate for this loss of balance.
Specific disease-associated isoforms and phosphorylation states of tau can lead to AD through the loss of mitochondria in axons. In studies of AD tissue, mitochondria have been found to be preferentially redistributed to the soma. These selective localizations can take place quickly, and are therefore difficult to quantify except by live videomicroscopy. In synapses, the mitochondria have been observed to be longer lived, and to play a more critical role in calcium regulation then those elsewhere. Disruption in the normal handling of calcium has been attributed to many aspects of AD, particularly synaptic pathology.
The canonical dogma that action potentials lead to vesicle fusion and transmitter release exclusively through the entry of extracellular calcium has recently been enhanced with the understanding that mitochondria contribute significantly to the synaptic calcium cycle. While mitochondria clearly do not depolarize as rapidly as whole spiking cells,(generally when mitochondria are depolarized there is some problem) their calcium transporters operate quickly to mop up and redistribute calcium. To say that mitochondria might single-handedly initiate vesicle fusion, or for that matter minipotentials or full-blown spikes, would await future experimental corroboration.
Countless scores of papers over the years have attempted to make sense of the myriad synaptic pathways underlying memory and LTP. They might be better understood when mitochondria are viewed as the primary authors of synaptic vesicle release probability, and by implication, “spontaneous” release (vesicle fusion in the absence of a spike). As in disease states, specific pathways, structures and organelles have significant roles to play in many aspects of brain function—but causally relating the motions and dynamics of mitochondria to these phenomena now gives the broadest interpretive power.