Fushun Yu

Effects of ageing and gender on contractile properties in human skeletal muscle and single fibres

Aim:  The objective of this study is to improve our understanding of the mechanisms underlying the ageing‐ and gender‐related muscle weakness.

Gender- and age-related differences in the regulatory influence of thyroid hormone on the contractility and myosin composition of single rat soleus muscle fibres

Abstract The effects of 4 weeks of thyroid hormone (3,5,3′-triiodothyronine, T3) treatment on the myosin isoform composition and maximum velocity of unloaded shortening (V0) of single soleus muscle fibres of young (3–6 months) and old (20–24 months) female (149 fibres) and male (200 fibres) rats were studied. Gender-related differences in the up-regulation of fast myosin heavy chain (MyHC) and myosin light chain (MyLC) isoforms were observed. In the female hyperthyroid rats, pure type I fibres and fibres co-expressing type I and type IIA MyHC (type I/IIA fibres) predominated. Some fibres expressed an α cardiac-like MyHC isoform either purely (α cardiac-like fibre type) or in co-expression with IIA MyHC (α cardiac-like/IIA fibre type). In the male hyperthyroid rats, on the other hand, all fibres were either type I/IIA or type I/IIAX. The relative quantities of fast MyLC isoforms in type I/IIA and type I/IIAX fibres was higher in female than in male hyperthyroid rats. V0 was similar in male and female control rats, and decreased with age in both genders (P<0.001). After T3 treatment, the average V0 increased (P<0.001) in females with a concomitant up-regulation of fast MyHC and fast MyLC isoforms irrespective of age. The average V0 of the pooled fibres was higher (P<0.001) in female than in male hyperthyroid rats at both ages. In conclusion, gender- and age-related differences were observed in the regulatory influence of 4 weeks’ T3 treatment on myosin isoform composition and V0 in soleus fibres. These differences are presumably related to an interaction of thyroid and sex hormones in the regulation of myosin gene expression.

Muscle cell and motor protein function in patients with a IIa myosin missense mutation (Glu-706 to Lys)

The pathogenic events leading to the progressive muscle weakness in patients with a E706K mutation in the head of the myosin heavy chain (MyHC) IIa were analyzed at the muscle cell and motor protein levels. Contractile properties were measured in single muscle fiber segments using the skinned fiber preparation and a single muscle fiber in vitro motility assay. A dramatic impairment in the function of the IIa MyHC isoform was observed at the motor protein level. At the single muscle fiber level, on the other hand, a general decrease was observed in the number of preparations where the specific criteria for acceptance were fulfilled irrespective of MyHC isoform expression. Our results provide evidence that the pathogenesis of the MyHC IIa E706K myopathy involves defective function of the mutated myosin as well as alterations in the structural integrity of all muscle cells irrespective of MyHC isoform expression.

Adaptation by alternative RNA splicing of slow troponin T isoforms in type 1 but not type 2 Charcot-Marie-Tooth disease

Slow troponin T (TnT) plays an indispensable role in skeletal muscle function. Alternative RNA splicing in the NH(2)-terminal region produces high-molecular-weight (HMW) and low-molecular-weight (LMW) isoforms of slow TnT. Normal adult slow muscle fibers express mainly HMW slow TnT. Charcot-Marie-Tooth disease (CMT) is a group of inherited peripheral polyneuropathies caused by various neuronal defects. We found in the present study that LMW slow TnT was significantly upregulated in demyelination form type 1 CMT (CMT1) but not axonal form type 2 CMT (CMT2) muscles. Contractility analysis showed an increased specific force in single fibers isolated from CMT1 but not CMT2 muscles compared with control muscles. However, an in vitro motility assay showed normal velocity of the myosin motor isolated from CMT1 and CMT2 muscle biopsies, consistent with their unchanged myosin isoform contents. Supporting a role of slow TnT isoform regulation in contractility change, LMW and HMW slow TnT isoforms showed differences in the molecular conformation in conserved central and COOH-terminal regions with changed binding affinity for troponin I and tropomyosin. In addition to providing a biochemical marker for the differential diagnosis of CMT, the upregulation of LMW slow TnT isoforms under the distinct pathophysiology of CMT1 demonstrates an adaptation of muscle function to neurological disorders by alternative splicing modification of myofilament proteins.

Genomic analysis of variation in hindlimb musculature of mice from the C57BL/6J and DBA/2J lineage.

The precise locations of attachment points of muscle to bone-the origin and insertion sites-are crucial anatomical and functional characteristics that influence locomotor performance. Mechanisms that control the development of these interactions between muscle, tendon, and bone are currently not well understood. In a subset of BXD recombinant inbred (RI) strains derived from the C57BL/6J and DBA/2J strains, we observed a soleus femoral attachment anomaly (SFAA) that was rare in both parental strains (Lionikas, Glover et al. 2006). The aim of the present study was to assess suitability of SFAA as a model to study the genetic mechanisms underlying variation in musculoskeletal anatomy. We scored the incidence of SFAA in 55 BXD strains (n = 9 to 136, median = 26, phenotyped animals per strain, for a total number of 2367). Seven strains (BXD1, 12, 38, 43, 48, 54, and 56) exhibited a high incidence of unilateral SFAA (47-89%), whereas 23 strains scored 0%. Exploration of the mechanisms underlying SFAA in 2 high incidence strains, BXD1 and BXD38, indicated that SFAA-relevant genes are to be found in both C57BL/6J and DBA/2J regions of the BXD1 genome. However, not all alleles relevant for the expression of the phenotype were shared between the 2 high-incidence BXD strains. In conclusion, the anatomical origin of the soleus muscle in mouse is controlled by a polygenic system. A panel of BXD RI strains is a useful tool in exploring the genetic mechanisms underlying SFAA and improving our understanding of musculoskeletal development.