Detection of myofibrillar proteins using a step gradient minigel with an ambiguous interface

Analysis of Titin in Red and White Muscles: Crucial Role on Muscle Contractions Using a Fish Model

Several studies have compared molecular components between red and white skeletal muscles in mammals. However, mammalian skeletal muscles are composed of mixed types of muscle fibers. In the current study, we analyzed and compared the distributions of titin, lipid, phosphate ions, and fatty acid levels in red and white muscles using a fish model (Tilapia), which is rich in red and white muscles, and these are well separated. Oil-red O staining showed that red muscle had more-abundant lipids than did white muscle. A time-of-flight secondary-ion mass spectrometric (TOF-SIMS) analysis revealed that red muscle possessed high levels of palmitic acid and oleic acid, but white muscle contained more phosphate ions. Moreover, elastica-van Gieson (EVG) and Mito-Tracker green FM staining showed that collagen and elastic fibers were highly, respectively, distributed in connective tissues and mitochondria in red muscle. An electron micrographic analysis indicated that red muscle had a relatively higher number of mitochondria and longer sarcomere lengths and Z-line widths, while myofibril diameters were thicker in white muscle. Myofibrillar proteins separated by SDS-PAGE showed that the major giant protein, titin, was highly expressed in white muscle than in red muscle. Furthermore, ratios of titin to myosin heavy chain (MHC) (titin/MHC) were about 1.3 times higher in white muscle than red muscle. We postulated that white muscle is fit for short and strong contractile performance due to high levels of titin and condensed sarcomeres, whereas red muscle is fit for low intensity and long-lasting activity due to high levels of lipids and mitochondria and long sarcomeres.

Comparative decline of the protein profiles of nebulin in response to denervation in skeletal muscle

The sliding filament model of the sarcomere was developed more than half a century ago. This model, consisting only of thin and thick filaments, has been efficacious in elucidating many, but not all, features of skeletal muscle. Work during the 1980s revealed the existence of two additional filaments: the giant filamentous proteins titin and nebulin. Nebulin, a giant myofibrillar protein, acts as a protein ruler to maintain the lattice arrays of thin filaments and plays a role in signal transduction and contractile regulation. However, the change of nebulin and its effect on thin filaments in denervation-induced atrophic muscle remains unclear. The purpose of this study is to examine the content and pattern of nebulin, myosin heavy chain (MHC), actin, and titin in innervated and denervated tibialis anterior (TA) muscles of rats using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), densitometry and electron microscopic (EM) analyses. The results revealed that denervation induced muscle atrophy is accompanied by decreased nebulin content in a time-dependent manner. For instant, the levels of nebulin in denervated muscles were markedly (P < 0.05) decreased, about 24.6% and 40.2% in comparison with innervated muscle after denervation of 28 and 56 days, respectively. The nebulin/MHC, nebulin/actin, and nebulin/titin ratios were decreased, suggesting a concomitant reduction of nebulin in denervated muscle. Moreover, a western blotting assay proved that nebulin declined faster than titin on 28 and 56 days of denervated muscle. In addition, EM study revealed that the disturbed arrangements of myofilaments and a disorganized contractile apparatus were also observed in denervated muscle. Overall, the present study provides evidence that nebulin is more sensitive to the effect of denervation than MHC, actin, and titin. Nebulin decline indeed resulted in disintegrate of thin filaments and shortening of sarcomeres.

Decline in titin content in rat skeletal muscle after denervation

Titin, an elastic and giant myofibrillar protein, is responsible for generating passive tension and maintaining sarcomere structure in striated muscles. Several studies have reported attenuation of passive tension and disorganization of sarcomere in atrophic muscles, but the changes of titin have not been investigated after denervation. For this purpose, we used sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunofluorescent staining to examine titin in innervated and denervated tibialis anterior (TA) muscles of the rat. With increasing denervation time, we found a greater loss of titin than myosin heavy chain (MHC) and actin contents in atrophic TA muscle. The ratios of titin/MHC and titin/actin gradually decreased following denervation. In contrast, ratios of MHC/actin in the denervated groups showed no significant differences with the controls even at 56 days postdenervation. The ultrastructure of myofibrils also showed disturbed arrangements of myofilaments and a disorganized contractile apparatus in denervated muscle. Immunofluorescent staining displayed translocation of the titin epitope from the Z-line to the I-band, suggesting that the apparent cleavage of titin occurred near the Z-line region during the atrophying process. Our study provides evidence that titin is more sensitive to degradation than MHC and actin after denervation. Moreover, the titin decline results in the loss of titin-based sarcomeric integrity in atrophic muscle.