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Machinery of the mind
LA JOLLA, Calif.—Scientists at The Scripps Research Institute (TSRI) have made several discoveries in recent months that shed light on the mechanisms underlying Parkinson’s disease and other neurodegenerative disorders. One study has determined the basic structural organization of a very complex molecular system whose defects are often linked with such disorders. Another series of studies from TSRI’s Florida campus has made progress toward using drug candidates to block a key pathway associated with cell death in Parkinson’s disease. Together, the discoveries represent significant steps toward understanding the causes of neurodegenerative disorders and development of effective diagnostics and therapies.
The large molecular motor known as the dynein-dynactin complex was the focus of a recent TSRI study based at the main campus in California, and TSRI notes that biologist have long sought to better understand the very complicated structural organization of this system. Dynein and dynactin normally work together on microtubules for cellular activities like cell division and intracellular transport of cargo, such as mitochondria and mRNA, and are key in neuronal development and repair. Problems with the dynein-dynactin motor system have been found in such brain diseases as Alzheimer’s, Parkinson’s and Huntington’s diseases and amyotrophic lateral sclerosis (ALS). Viruses such as herpes, rabies and HIV also appear to hijack the dynein-dynactin transport system to get deep inside cells.
Previous research has illuminated the structure of specific pieces of the dynein-dynactin complex, but its large size, myriad subunits and high flexibility have previously prevented scientists from gaining a clear picture of the whole system. For the first time, scientists at TSRI were able to create a two-dimensional visualization of the complex as a whole.
“This is a significant and fundamental step towards understanding this complex system, which plays a critical role in clearing away aggregation out of the branches of the cell body,” Gabriel Lander, a TSRI biologist, tells DDNews. “The more we can understand about how nutrients are moved along neurons, the more capable we are of developing ways to treat neurodegenerative disorders.”
Lander’s laboratory was able to map out the dynein-dynactin complex by using electron microscopy (EM) on sample dynein and dynactin proteins produced from cow brains at the laboratory of Trina A. Schroer at Johns Hopkins University. These two proteins, although they are very complicated even when they are not linked together, have been so highly conserved by evolution that they are found in almost identical forms in many organisms.
TSRI scientists were able to develop two-dimensional images of these dynein and dynactin molecules with unprecedented detail and reveal subunits that had never before been observed. They then developed a strategy to purify and image dynein and dynactin in complex together on a microtubule—a railway- like structure, ubiquitous in cells, along which dynein-dynactin moves its cargoes. “We had to collect a great deal of data to get a clear picture of such a complex system,” Lander explains. “Typically, people using this sort of technique collect a couple hundred images, but for the whole system we had to collect about 24,000.”
The findings of the research, which have been published online by Nature Structural & Molecular Biology, clarify how dynein and dynactin fit together on a microtubule, how they recruit cargoes and how they manage to move those cargoes consistently in a single direction.
Lander tells DDNews that his lab will now attempt to produce a higher-resolution, three-dimensional image of the dynein-dynactin-microtubule complex using an EM-related technique called electron tomography.
As far as the news from Florida, scientists from that TSRI campus have released findings that shed light on another group of molecules that are closely linked to the development of Parkinson’s disease: JNK kinases. In a pair of related studies, they were able to demonstrate that their drug candidates could target these enzymes, which are linked to the destruction of brain cells in Parkinson’s disease and possibly in other neurodegenerative diseases. The findings raise hope that a therapy could use such targeting to prevent the gradual degeneration of brain cells in Parkinson’s disease and halt patients’ decline. The studies were published in the Journal of Medicinal Chemistry and Scientific Reports.
The drug candidates that were part of the studies act on the kinases known as JNK1, JNK2 and JNK3, each of which has a unique biological function. They are linked to many of the hallmark components of Parkinson’s disease, such as oxidative stress and programmed cell death. The scientists' success at targeting these enzymes suggest that it is possible to design drug candidates that are both highly effective and highly selective in fulfilling their function to protect mitochondria, which provide the cell with energy and can ultimately prevent brain cell death.
“These are the first isoform selective JNK 2/3 inhibitors that can penetrate the brain and the first shown to be active in functional cell-based tests that measure mitochondrial dysfunction,” said Philip LoGrasso, a TSRI professor who led both studies. “In terms of their potential use as therapeutics, they’ve been optimized in every way but one—their oral bioavailability. That’s what we’re working on now.”
The scientists found that within JNK3, a single amino acid—L144—was primarily responsible for the high level of JNK3 selectivity. Isoform selectivity can help to limit potential side effects of a drug. This may prove to be an especially valuable insight, because some recent studies have shown that JNK3 plays a central role in brain cell death not only in Parkinson’s disease, but also in Alzheimer’s disease. LoGrasso and his colleagues also believe their JNK3 drug candidates have potential for treating ALS.