Models of Epsilon-Sarcoglycan Gene Inactivation and their Implications for the Pathology of Myoclonus Dystonia

Title: Models of Epsilon-Sarcoglycan Gene Inactivation and their Implications for the Pathology of Myoclonus Dystonia
Authors: Given, Alexis
Date: 2013
Abstract: Myoclonus Dystonia (MD) is an autosomal dominant movement disorder characterized by bilateral myoclonic jerks paired with dystonia 1. Mutations have been mapped to the ε-sarcoglycan (SGCE) gene in about 40% of patients 2,92. The purpose of this project was to examine the properties of SGCE in the central nervous system (CNS) and use this knowledge to elucidate the pathology of MD. Although Sgce is a member of the sarcoglycan complex (SGC) in other tissues, little is known about its interactions in the CNS. The vast majority of mutations in SGCE alter the translational reading frame. Proteins arising from these rare mutations are less stable than the wild type (WT) and undergo preferential degradation via the ubiquitin proteasome system 3. As this locus is maternally imprinted, patients with MD are effectively null for sgce expression 73,91. Therefore, Sgce knock out (KO) models should approximate MD conditions both in vivo and in vitro. As there are no current treatments for MD, in sight into the pathology of the disease will aid in eventual treatments and help bring patients some relief by finally understanding their disease. Since a large percentage of MD patients are without the sgce protein, identifying what this protein’s function is and how its absence effects normal processing in the brain should help to identify the underlying cellular pathology which produces the MD phenotype. This research was performed under the hypothesis that, in neuronal cells, sgce interacts with a group of proteins that together play a role in stabilization and localization of ion channels and signaling proteins at the cell membrane. The aims were to: (1) Build a MD mouse model with either a conditional knock-out (cKO) or a conditional gene repair (cGR) mutation; (2) Use neuroblastoma cells to identify the other proteins which interact with sgce in neurons, and; (3) Determine if there is a disruption of the localization of the sgce-complex members due to the loss of sgce. Recombineering was used to complete the constructs for two transgenic mouse models: One model for the KO of exon 4 of sgce and one for the cGR in intron 1. Primary neurosphere lines from two previously generated chimeras were developed, as well as from a WT mouse. These neurosphere cell lines allowed comparisons of RT-PCR results from a heterogeneous neurological cell population to neuroblastoma cell lines. mRNA is present in neuronal cells for many of the DGC associated proteins. It was confirmed that the KD of sgce results in a reduction of nNOS protein and in increased proliferation of NIE cells. By using a nitrite/nitrate assay as well as studies with L-NAME, it was confirmed that this increased proliferation was in fact due to a lack of nNOS function. These proliferation changes did not occur in N2A cells, which do not express high levels of nNOS during proliferation, further confirming nNOS’s role in the proliferation changes. Using qRT-PCR, KD of sgce was shown to result in significant changes in the transcript levels for many DGC associated proteins. This suggests that a DGC-like complex is forming in neuronal cells. Also, as a result of difficulties with the research, it became clear that over-expression of sgce causes cell death. This observation was quantified using cell counts and TUNEL staining, both showing significant results. Additionally, several new constructs were created which will hopefully be of use for future students wanting to study sgce’s functions. New shRNA targeting sgce and sgcb have been made and both constructs result in reducing the expression of sgce. Seven different flag-tagged sgces have been created and some of these have been transferred into a tet-inducible system, which should circumvent the problem of over-expression. Finally GFP-tagged constructs for sgce and sgcb have been made and pooled clones have been developed. These tools will hopefully enable future students to continue to tease apart sgce’s function(s).
CollectionThèses, 2011 - // Theses, 2011 -