Biology subjects

Kalakoutis, M.

Publications and source records attributed to Kalakoutis, M..

4 recordsLinked to original sources

bioRxiv2026

The molecular structural basis of the braking action of muscle

Skeletal muscles generate force and shortening through a structural powerstroke in actin-bound myosin motors linked to ATP hydrolysis, but they also act as brakes to resist stretching by an external load. The molecular structural basis of muscle braking has remained obscure. Here we show that muscle braking is driven both by structural changes in myosin motors and by distortion of the lattice of thick and thin filaments including their M-band and Z-disk connections. A strained motor conformation is established early during stretch, but force continues to increase as thin filaments are displaced from their symmetrical lattice positions, the lateral shear between thick filaments increases, and the lattice volume decreases. Moreover, whilst the motor conformation rapidly recovers after the stretch, the distortion of the filament lattice is largely maintained, storing the work done on the muscle during stretch and accounting for the residual force enhancement after stretch.

biophysics↗

bioRxiv2025

Distinct distributions of myosin motor conformations during contraction of slow and fast skeletal muscle

Slow skeletal muscles maintain posture and produce graded movement at low metabolic cost. Force development and ATP utilisation during fixed-end contractions are typically five times slower in slow than fast muscles from the same species. Mechanical measurements previously suggested that more myosins are attached to thin filaments during contraction of slow muscle, which seems incompatible with its high efficiency. We therefore used small-angle X-ray diffraction to provide a structural estimate of the fraction of myosins attached to thin filaments in slow muscle. X-ray signals associated with myosin binding to actin indicate that only about 10% of myosin motors are actin-bound during fixed-end tetani of rat soleus slow muscles, compared with about 25% in mouse EDL fast muscle. Moreover, X-ray signals associated with the helical organisation of OFF myosin motors in the thick filaments show that about 70% of myosin motors remain in the OFF conformation during tetanic contraction of slow muscle, compared with only 30% in fast muscle. The much slower force development in soleus muscle also allowed clear separation of early structural changes in thick filaments on activation, some of which are distinct from those reported previously in fast muscles. Moreover, the early structural changes in soleus muscle have about the same amplitude in a twitch and a tetanus, suggesting that they are triggered by thin filament activation rather than thick filament stress, and implying a fast signalling pathway between thin and thick filaments. Key PointsO_LIThe interaction between myosin motors and actin filaments in slow skeletal muscles maintain posture and produce graded movement at low metabolic cost. C_LIO_LIMechanical studies have suggested that more myosins are attached to actin filaments in slow than in fast muscle, but this seems incompatible with its high efficiency. C_LIO_LIWe used X-ray diffraction to show that there are fewer myosin motors attached to actin in slow muscle than in fast muscle because more motors are sequestered on the myosin filament. C_LIO_LIThe slower force development in slow muscle also allowed us to isolate and characterise fast changes in myosin motor conformation associated with activation of the actin filaments. C_LIO_LIThe results reveal a distinct pathway of inter-filament signalling in slow muscle that could help the development of novel therapies for muscle weakness. C_LI

biophysics↗

bioRxiv2023

Muscle fibre size and myonuclear positioning in trained and aged humans

Myonuclear domain (MND) is the theoretical volume of cytoplasm within which a myonucleus is responsible for transcribing DNA. Changes in myonuclear number, organisation, and myonuclear domain size are associated with exercise adaptations and ageing. However, data on satellite cell activation, changes in MND volumes and myonuclear arrangement following exercise are inconsistent. Additionally, whether MNDs and myonuclear arrangement are altered with age remains unclear. The aim of the present investigation was therefore to investigate relationships between age and activity status and myonuclear numbers and organisation. Muscle fibres from younger trained (YT) and older trained (OT) individuals were compared with age-matched untrained counterparts (YU and OU). Serial, optical z-slices were acquired throughout isolated muscle fibres and analysed to give 3D coordinates for myonuclei and muscle fibre dimensions, respectively. As expected, mean cross-sectional area (CSA) (m2) of muscle fibres from OU was 29-42% smaller compared to the other groups. Number of nuclei relative to fibre CSA was 87% greater in OU compared to YU muscle fibres (P < 0.05). Additionally, scaling of myonuclear domain volume with fibre size was altered in older untrained individuals. Myonuclear arrangement, on the other hand, was similar across groups. These data indicate that regular endurance exercise throughout the lifespan may preserve the size of single muscle fibres in older age and maintain the relationship between fibre size and MND volumes. Inactivity, however, may result in reduced muscle fibre size and disrupted relationship between fibre size and MND volumes. Plain Language SummaryIn this study, we examined the relationship between physical activity and the characteristics of muscle fibres in individuals of different age groups. We focused on a concept called the myonuclear domain (MND), which refers to the volume surrounding muscle nuclei or myonuclei that house the genome. We wanted to understand how changes in myonuclear number, organisation, and MND size were influenced by exercise and aging. To do this, we compared muscle fibres from younger trained individuals, older trained individuals, and age-matched untrained individuals. The results showed that the average size of muscle fibres in the untrained older individuals was smaller compared to the other groups. Moreover, the number of nuclei relative to fibre size was significantly higher in the untrained older individuals. However, myonuclear arrangement was similar across all groups. These findings suggest that regular endurance exercise throughout life may help maintain muscle fibre size, myonuclear numbers, MND volumes, and myonuclear organisation in older individuals. Conversely, inactivity can lead to reduced muscle fibre size and disrupted relationship between fibre size and MND volumes. These results have important implications for understanding the effects of exercise and aging on muscle health.

physiology↗

bioRxiv2022

Exercise induces myonuclear remodelling in humans independently of age

Age-related decline in skeletal muscle structure and function can be mitigated by regular exercise. However, the precise mechanisms that govern this are not fully understood. The nucleus plays an active role in translating forces into biochemical signals (mechanotransduction), with nuclear lamina protein Lamin A regulating nuclear shape, nuclear mechanics, and ultimately gene expression. Defective Lamin A expression causes muscle pathologies and premature ageing syndromes, but the roles of nuclear structure and function in physiological ageing and in exercise adaptations remain obscure. Here, we isolated single muscle fibres and carried out detailed morphological and functional analyses on myonuclei from young and older exercise-trained individuals. Strikingly, myonuclei from trained individuals were more spherical, less deformable, and contained a thicker nuclear lamina than untrained individuals. Complementary to this, exercise resulted in increased levels of Lamin A and increased myonuclear stiffness in mice. We conclude that exercise is associated with myonuclear remodelling, independently of age, which may contribute to the preservative effects of exercise on muscle function throughout the lifespan. Key pointsO_LIThe nucleus plays an active role in translating forces into biochemical signals C_LIO_LIMyonuclear aberrations in a group of muscular dystrophies called laminopathies suggest that the shape and mechanical properties of myonuclei are important for maintaining muscle function. C_LIO_LIHere, we present striking differences in myonuclear shape and mechanics associated with exercise, in both young and old humans. C_LIO_LIMyonuclei from trained individuals were more spherical, less deformable, and contained a thicker nuclear lamina than untrained individuals. C_LIO_LIWe conclude that exercise is associated with age-independent myonuclear remodelling, which may help to maintain muscle function throughout the lifespan. C_LI

cell biology↗