The aim of this work was to describe the role of the protein neurofibromin in skeletal muscle cells. Loss of neurofibromin function causes neurofibromatosis I (NF1)—one of the most common genetic disorders worldwide. Although the most prominent symptoms of NF1 are tumours of neuroectodermal origin, a high fraction of patients suffers from pathological changes of the musculoskeletal system. These symptoms mostly appear in early childhood and include bone malformations, decreased bone density and a high predisposition to fractures. An additional reduction of muscular strength and coordination skills increases the risk of falls and injuries which altogether contributes to tremendous restrictions of patients' health and life quality.
Neurofibromatosis I is caused by mutations in the gene NF1. The gene encodes for the RAS GTPase activating protein neurofibromin, which is described as a tumour suppressor and histogenesis control gene. Although considerable research was performed on tumourgenesis and skeletogenesis in NF1 patients, the aspect of putative muscular defects has not been investigated yet. However, in respect to skeletal malformations it seems most likely that a combination of skeletal and muscular defects causes the clinical picture. Therefore, basic pathomechanisms of NF1 in muscle cells were studied in this work. For this purpose, three mouse models were generated using a conditional knockout approach allowing a muscle cell specific Nf1 knockout. In each model Nf1 was deleted in a specific stage of embryonic development. In the first model (Nf1Lbx1) Nf1 was deleted in muscle precursor cells. In the second model (Nf1Myf5) myoblasts were targeted and in the third model (Nf1HSA) Nf1 was deleted in myotubes. The mice were analysed using histological, immunochemical, molecular biological and biochemical methods. Thereby, the four phases of muscle development were analysed: embryonic, fetal, perinatal and adult myogenesis.
It could be shown that loss of Nf1 causes a decrease in muscle fibre growth. This decrease was first detected in the fetal phase of myogenesis at embryonic day E18.5. It was detected only if Nf1 was deleted in precursor cells or myoblasts, but not if deleted in myotubes. The fibre size reduction was mainly due to reduction of the myonuclear domain (cellular protein content per nucleus). Furthermore, it was detected that the number of Pax7+ progenitor cells showed a transient increase in the fetal stage upon deletion of Nf1. However, postnatally the Nf1 deletion resulted in strong depletion of Pax7+ progenitor cell numbers. That leads to a diminished cell pool of muscle stem cells, so called satellite cells, which are responsible for muscle regeneration. Also, it seems that Nf1 has a role in fibre type determination and regulation of metabolic parameters as loss of Nf1 caused a shift towards oxidative fibre characteristics. Investigations of molecular pathways in muscle tissue showed that deletion of Nf1 leads to significant over-activation of the Ras downstream components Erk/Mek, Akt and Nfat as well as transcriptional upregulation of their downstream targets which might be responsible for the detected growth and metabolism defects.
The results show that Nf1 has several roles in skeletal muscle cells, including differentiation of Pax7+ precursor cells, fibre growth and regulation of metabolic characteristics. The described mouse models recapitulated aspects of neurofibromatosis I associated muscle defects like the decrease of muscle size. Therefore, they are suitable for further studies and therapy testing in the process of neurofibromatosis I research.
|Commitee:||Lauster, Roland, Stricker, Sigmar|
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Keywords:||Tumour suppressor, Nf1, Growth, Metabolism, Skeletal muscle cells|
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