Author(s): VAN BELLEGHEM, K., ENGMAN, V., HIAM, D., BROWNE, J., LAMON, S., Institution: DEAKIN UNIVERSITY, Country: AUSTRALIA, Abstract-ID: 1217

INTRODUCTION:
Sex differences in muscle structure and function are well documented, but in humans, the intertwined nature of sex chromosomes and gonadal hormones makes it difficult to determine their individual contributions. The Four Core Genotypes (FCG) rat model uniquely allows separation of sex chromosome complement from gonadal type (XX and XY animals with testes, XX and XY animals with ovaries), providing a powerful tool to dissect their independent effects on muscle physiology. This study provides a characterization of muscle morphology, transcriptomic profiles, and intramuscular sex hormone levels in the FCG rat model.
METHODS:
Tibialis anterior muscles from the Four Core Genotypes (FCG) rat model — XX with ovaries (XXf, n = 10), XX with testes (XXm, n = 10), XY with ovaries (XYf, n = 4), XXY with ovaries (XXYf, n = 8), and XY with testes (XYm, n = 10) — were collected for immunohistochemistry, RNA sequencing, and intramuscular sex hormone measurements to assess fibre morphology, gene expression, and hormonal profiles.
RESULTS:
Skeletal muscle cross-sectional area (CSA) differed significantly between genotypes. Total tibialis anterior CSA was smaller in XXYf rats compared with XXm and XYm rats (p < 0.05). Fibre-type–specific analyses revealed no difference between genotypes in type 1 fibre size, whereas type 2a, type 2x, and type 2b fibre size exhibited robust genotype-dependent differences (all p < 0.001). Type 2a fibres were smaller in XXf compared with XXYf, XXm, and XYm rats, while type 2x and type 2b fibres were generally larger in rats with testes (XYm and XXm) compared with XXf and XXYf rats. Fibre-type composition was largely similar across genotypes, with the exception of type 1 fibres, which were present at a higher proportion in XXYf rats compared with XXm and XYm rats (p < 0.05).
Preliminary transcriptomic analysis revealed clear separation along PC1 (23.7%), largely reflecting gonadal phenotype (testes vs ovaries), with less distinct segregation by sex chromosome complement (XX vs XY). Differential gene expression analyses identified between 11 and 2,342 DEGs across pairwise comparisons, with the largest differences observed between XXf vs XXm (2,342 DEGs) and XXf vs XYm (1,832 DEGs).
Ongoing analyses will include intramuscular sex steroid hormone levels across genotypes to further contextualize these findings.
CONCLUSION:
These findings highlight the interplay between sex chromosome complement, gonadal type, and muscle structure, providing a foundation for further investigation into the molecular mechanisms underlying sex differences in skeletal muscle.