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Genetic Mechanisms of Muscle Fibre Variation

Final Report Summary - GENETMUSCLEFIBREVAR (Genetic mechanisms of muscle fibre variation)

An adequate amount of muscle is required to fulfil its vital functions such as locomotion, respiration, thermoregulation, maintenance of glucose homeostasis and support and protection of internal organs and bones. Muscle loss during aging (known as sarcopenia), or as a result of diseases such as muscular dystrophy, compromise these functions and lead to deterioration of the quality of life. Between 9 and 18 % of individuals over 65 years of age suffer from sarcopenia, and it is going to be an increasingly important healthcare issue with the growing population of the elderly.

Muscle mass is determined by the number and size of the muscle fibres, the specialized mature cells of the muscle tissue. There are substantial differences among individuals in the number and / or size of muscle fibres constituting individual muscles. Muscle loss due to aging or disease most severely affects those with the fewer and smaller fibres.

Approximately half of the variation in muscle mass is hereditary, indicating that individuals can have greater or smaller muscle mass because they inherited certain variants of the relevant genes from their parents. Identity of such genes is of particular interest because they could offer targets for development of the treatment for muscle wasting conditions. Yet only a limited number of genes responsible for the variation in muscle mass are currently known.

Model organisms play an important role in improving our understanding of the cellular mechanisms of the structure and function of skeletal muscle because both the size and the number of muscle fibres can be assessed. From the studies in mammals it appears that common mechanisms are involved in the regulation of muscle properties in different species. For instance, mutations in the myostatin gene lead to a similar hypermuscularity phenotype in mice, dogs, cattle and humans. Thus the use of the model organisms can be very valuable with respect to translation of the results to humans. The mouse model has been particularly useful for studying the genetic mechanisms because it allows integration of the data of the gross phenotypes such as weight, cellular characteristics such as number and size of the fibres, as well as the transcriptional and genomic information. The present project aimed to explore the genetic mechanisms underlying the primary determinants of muscle mass, the number and size of the fibres in the mouse. In particular, we focused on the soleus muscle which in mice encompasses the same types of fibres, type 1 and type 2A, found human muscles.

We first asked what the role of genes was in determining the properties of muscle fibres in the mouse soleus. To address that, we examined the number, size and proportion of the type 1 fibres in eleven strains of laboratory mice. Each strain contains a unique collection of gene variants captured from the pool of variants present in the species, and the between strain difference signals of the importance of genes in determining trait of interest. We found that the number of fibres can differ by more than twofold, between 500 and 1200 approximately, in soleus of different strains. Because the number of fibres remains largely unchanged after birth, the role of genes is particularly important. The size of the fibres, measured as the cross-sectional area, contrasted even more substantially as both type 1 and type 2A fibres differed by approximately threefold among the strains. The number and the cross sectional area of the fibres appeared not inseparably linked indices; there were strains with the similar number but different size of the fibres, and with a similar size but different number of fibres. The proportion of type 1 fibres can also contribute to the variation in muscle mass because often there is a difference in size between the type 1 and type 2A. We found that the proportion of type 1 fibres ranged from 25 to 64 % approximately amongst the strains. Altogether, these findings revealed that muscle mass can be achieved in different ways: through the number, the size and / or proportion of different fibre types. Furthermore, a substantial role of the genetic factors emerged in determining these indices. Importantly, the studies identified suitable models for the search of genes involved in determining the size or the number of fibres. In the next phase of the project we initiated the search for specific genes affecting properties of muscle fibres. It has been carried out by means of the association analysis which allows identification of the link between the regions of the genome known as quantitative trait loci (QTL) and the trait of interest in a population derived by crossing for several generations of two of the examined strains which differed twofold in the size of the fibres and in the proportion of the type 1 fibres. We measured fibre properties in more than 450 solei approximately equally divided between males and females. Such large sample permitted us to carry out statistically robust comparisons between sexes. It emerged that similar number of fibres is present in males and females, whereas the size of the fibres was larger in males. Thus the larger muscle mass in males is primarily due to the size but not the number of fibres. Consistently with the observations in humans we also found that female muscles had a larger proportion of type 1 fibres compared to males, on average by approximately 7 percentage points. We then carried out the association analysis and identified a number of QTL which contributed to the difference in fibre size and proportion of type 1 fibres between the two strains. These QTL cover genomic regions which harbour more than one gene, and therefore further studies will be required in order to identify the causative genes. Genomic regions in pig harbouring homologous genes were also implicated in the regulation of the size of the fibres further suggesting the similarity of the mechanisms amongst different species.

In conclusion, we found that genes are playing an important role in determining muscle mass via regulation of the number, size and proportion of different fibre types. In addition, we identified the models that can be used for the search of genes affecting those traits and initiated such search in one of them. Identification of the specific genes in the future will open new targets for the pharmacological agents to combat muscle loss. The livestock breeders will also benefit from this information which will enable them to develop breeds with favourable meet quality and better yield.

The project played an important role in further development of the research agenda of the PI, contributed to teaching strength, fostered collaborative ties with the European and third countries and facilitated his integration at the University of Aberdeen which is completed now. The established research programme will contribute to European excellence and competitiveness.