1. Huntingtin in skeletal muscle and its effects on mitochondrial respiration in the R6/2 transgenic mouse and Huntington disease mutation carriers
- Author
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Lindenberg, Katrin S., Ludolph, Albert C., and Iben, Sebastian
- Subjects
Mitochondrium ,Skelettmuskel ,Huntington-Chorea ,ddc:610 ,Huntington disease ,Atmungskette ,Muscle, Skeletal ,DDC 610 / Medicine & health ,Mitochondria ,Metabolism ,Mitochondrial respiration - Abstract
Huntington disease (HD) is a genetic neurodegenerative disorder caused by an expanded CAG trinucleotide repeat expansion in the first exon of the Huntingtin (HTT) gene. HTT is widely expressed in neuronal and non-neuronal tissue. Beside the typical movement disorder phenotype and progressive cognitive impairment many HD patients display progressive weight loss and muscular atrophy despite a high caloric intake. This phenotype can even occur in early symptomatic HD mutation carriers and is related to an increased metabolic rate in HD. The underlying mechanisms for this catabolic state are still unknown. Several studies describe diverse effects of mutant HTT on mitochondrial morphology and function. The demand for cellular energy supply differs between organs, therefore it is likely that mutant HTT has distinct effects on mitochondrial respiration in different organs. Skeletal muscle tissue is an easily accessible tissue for small biopsies and it is often considered for the development of molecular biomarkers for monitoring disease progression or as a surrogate marker for therapeutic interventions in HD. A prerequisite for the development of biomarkers would be a better understanding of the differences and commonalities in alterations of metabolic changes in brain and skeletal muscle in HD. Therefore some aspects of alterations in skeletal muscle were investigated in the widely used and well-characterized transgenic (tg) R6/2-mouse model for HD. In enzyme histological staining suitable to detect mitochondrial defects in skeletal muscle, no obvious morphological differences between transgenic and wild type (wt) mice were observed. Thus, so far, the localization and function of HTT in skeletal muscle is not known. Against this background skeletal muscle tissue from wt and tg mice were used for immunofluorescence staining with anti-HTT antibodies and for subfractionation by differential centrifugation with subsequent Western blot analysis in this thesis. A more intense immunofluorescence staining for HTT in the fast-twitch fibers type IIa suggests a higher expression of HTT in this subset of muscle fibers. The results from skeletal muscle subfractionation point out that HTT is mainly associated with the sarcolemma and sarcoplasmic reticulum. The limitation of this qualitative not quantitative analysis is that the differentiation of the subfractions needs to be improved for a higher purity. For functional respirometric analysis of mitochondria in brain and soleus muscle an oxygraph was used. Mitochondrial respiration was increased in prefrontal cortex of tg mice compared to wt. In contrast, mitochondrial activity was decreased in soleus muscle of tg mice in comparison to wt littermates. This suggests that mutant HTT has diverse and even opposite effects on mitochondrial respiration in addition to known differences in metabolic demands and mitochondrial activity of different organs. As the skeletal muscle is early affected in HD and easily accessible it is still an interesting tissue for studying the metabolic changes of the disease. In the EINSTEIN- feasibility trial the response of mitochondrial respiration after 3 hours of an acute bout of exercise on a cycling ergometer was investigated in fine needle muscle biopsies of 8 presymptomatic HD mutation carriers in comparison to 8 healthy controls. First, all participants completed the trial and no biopsy-related adverse events occurred. No differences in mitochondrial respiratory activity in the vastus lateralis muscle were found between pre-symptomatic HD-mutation carriers and healthy controls after physical exercise. However, slight differences were found in the response of mitochondrial respiration to physical exercise between the two groups. This suggests that in principle this technique in conjunction with a physical exercise test seems promising to detect early functional changes related to mitochondrial respiration in the skeletal muscle of pre-symptomatic HD-mutation carriers. In conclusion, a better understanding of mutant HTT in skeletal muscle and its effects on metabolism can contribute to explore pathophysiological changes in HD. However, the molecular and biochemical signatures of HD needs to be considered and encountered in an organ-specific way.
- Published
- 2023
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