Alterations in thin filament length during postnatal skeletal muscle development and aging in mice

Abstract

The lengths of the sarcomeric thin filaments vary in a skeletal muscle-specific manner and help specify the physiological properties of skeletal muscle. Since the extent of overlap between the thin and thick filaments determines the amount of contractile force that a sarcomere can actively produce, thin filament lengths are accurate predictors of muscle-specific sarcomere length-tension relationships and sarcomere operating length ranges. However, the striking uniformity of thin filament lengths within sarcomeres, specified during myofibril assembly, has led to the widely held assumption that thin filament lengths remain constant throughout an organism's lifespan. Here, we rigorously tested this assumption by using computational super-resolution image analysis of confocal fluorescence images to explore the effects of postnatal development and aging on thin filament length in mice. We found that thin filaments shorten in postnatal tibialis anterior (TA) and gastrocnemius muscles between postnatal days 7 and 21, consistent with the developmental program of myosin heavy chain (MHC) gene expression in this interval. By contrast, thin filament lengths in TA and extensor digitorum longus (EDL) muscles remained constant between 2 mo and 2 yr of age, while thin filament lengths in soleus muscle became shorter, suggestive of a slow-muscle-specific mechanism of thin filament destabilization associated with aging. Collectively, these data are the first to show that thin filament lengths change as part of normal skeletal muscle development and aging, motivating future investigations into the cellular and molecular mechanisms underlying thin filament adaptation across the lifespan.

Keywords: actin; mouse; myofibril; myofilament; sarcomere; tropomodulin.

Figure 1

Sample immunofluorescence images of P14 mouse muscles. Longitudinal TA cryosections were phalloidin-stained for F-actin and immunostained for α-actinin, along with either Tmod1 or nebulin M1M2M3, and imaged by confocal microscopy. P, thin filament pointed ends; Z, Z-line; H, H-zone. Bars, 1 μm.

Figure 2

Thin filament lengths during postnatal muscle development in mice. Graphs depict distances of Tmod (solid circles) and nebulin M1M2M3 (open circles) from the Z-line in GAS and TA muscles from P7 to P21 mice. Tmod distance reflects the variable position of the thin filament pointed end with respect to the Z-line, while nebulin M1M2M3 distance reflects the relatively constant position of the proximal/distal segment boundary of the thin filament with respect to the Z-line (Gokhin and Fowler, 2013). Tmod distances reflect pooled Tmod1 and Tmod4 distances. Error bars reflect mean ± SD for n ≥ 100 myofibrils/time-point for Tmod, n ≥ 50 myofibrils/time-point for nebulin M1M2M3. ^*p < 0.01 with respect to the previous time-point.

Figure 3

Thin filament lengths in aged mouse muscles. Graphs depict distances of Tmod (solid circles) and nebulin M1M2M3 (open circles) from the Z-line in TA, EDL, and soleus muscles from 2-mo-old to 2-yr-old mice. Tmod distance reflects the variable position of the thin filament pointed end with respect to the Z-line, indicative of variations in length, while nebulin M1M2M3 distance reflects the relatively constant position of the proximal/distal segment boundary of the thin filament with respect to the Z-line (Gokhin and Fowler, 2013). Tmod distances reflect pooled Tmod1 and Tmod4 distances. Error bars reflect mean ± SD for n ≥ 100 myofibrils/time-point for Tmod, n ≥ 50 myofibrils/time-point for nebulin M1M2M3. ^*p < 0.01 with respect to the previous time-point.