RNA binding proteins and epigenetics in SCA7

Abstract: Polyglutamine diseases are a group of nine disorders that includes, among others SCA7. The common denominator is an expanded glutamine tract in the respective disease protein caused by unstable replication during meiosis. Most research within this field points to a combination of gain-of-function and loss-of-function mechanisms causing all polyglutamine diseases. Using a SCA7 model we are thus attempting to study both of these mechanisms. The glutamine tract expansion responsible for SCA7 is located in the protein Ataxin-7, which like the other polyglutamine proteins aggregates into large inclusions in patient cells. In a gain-of-function mechanism, the aggregates are suggested to cause stress to the cell by e.g. sequestering vital proteins into the aggregates, which could disrupt their function. RNA-binding proteins such as FUS and TDP-43 are often found in aggregates in neurodegenerative diseases, and have been observed in SCA7 aggregates as well. However, if disruption of FUS and TDP-43 function occurs, or if it plays a role in SCA7 pathology is unclear. We found a high rate of co-aggregation of FUS with Ataxin-7 using immunofluorescence and filter trap assays. Furthermore, we found that both the localization and function of FUS was altered in a SCA7 cell model using cell fractionations and RT-PCR. Additionally, we found that TDP-43 also co-aggregated with Ataxin-7 and phosphorylation of TDP-43 was increased during the disease phenotype.Wild-type Ataxin-7 normally functions within chromatin regulation processes, and loss-of-function pathology in SCA7 could therefore involve a disruption of these processes. We have developed a method, FRIC, that enables us to study chromatin organization in live cells using confocal microscopy and fluorescently tagged histones. Using inhibitors of HATs and HDACs, as well as a previously known protein that regulates chromatin structure, we were able to observe changes in chromatin structure in the nuclear periphery, confirming the usefulness of FRIC. Additionally, we investigated the involvement of an inner nuclear membrane protein, Samp1, in chromatin organization and found Samp1 to be instrumental in organizing peripheral chromatin.Taken together, the results from these two studies indicate that SCA7 pathology disturbs RNA-binding protein mediated transcriptional regulation in a gain-of-function mechanism, and that FRIC is a powerful new tool for examining chromatin regulation in diseases with disrupted transcription, like SCA7.