Myonuclear degeneration in LMNA null mice

Abstract

Lamins A/C, the major constituent of the nuclear lamina, confer mechanical stability to nuclei. We examined the myonuclei of LMNA null mice at the myotendinous junctions (MTJ), the site of longitudinal force transmission from contractile proteins to extracellular proteins. The right soleus and rectus femoris muscles of five null mutants aged 5-7 weeks and two wild-type animals aged 5 weeks and 6 months were examined by electron microscopy. The myofibers merging into the tendons were assessed for nuclear disintegration and cytoplasmic degeneration. The myofibers of the wild-type rectus femoris and soleus muscles revealed 19-27 nuclei/50 myofibers and 5-8/20, respectively, with no signs of degeneration. The rectus femoris muscle fibers of the null mice contained 75-117 myonuclei/50 myofibers, the soleus muscle, 13-36 nuclei/20 myofibers. Eleven to twenty-one per 50 myonuclei of the rectus femoris myonuclei showed chromatin clumping, nuclear fragmentation, nuclear inclusions and invaginations, and intranuclear filaments. The values were 12-19/50 for the soleus myonuclei. Moreover, 5-12/50 rectus femoris myofibers and 5-14/20, of the soleus myofibers showed cytoplasmic degeneration. None of these changes was found distal to the MTJ. These results favor the notion that myonuclei lacking a functional lamina are susceptible to mechanical stress in vivo. These alterations may contribute to the development of early joint contractures, a feature of ADEDMD.

Figure 1

Myofibers of the rectus femoris muscle facing the tendon. A, B. Null mouse. A. The number of myonuclei per myofiber is increased in muscle adjacent to tendon. B. Higher magnification. C, D. Wild‐type mouse. C. This was not seen in the wild‐type mouse. D. The electron micrograph shows the junctional folds of a myofiber merging with the tendon. Bars, 10 µm.

Figure 2

A. Normal myonucleus of the wild‐type mouse. B–J. The spectrum of myonuclear changes in null mutants. B. Condensed chromatin, surrounded by a felt‐like rim. C. Condensed chromatin, surrounded by concentric lamellae. D. Fragmented nucleus containing tubular profiles (arrow). E. Condensed fragmented chromatin. F. Condensed fragmented chromatin with inclusions. Note nuclear membrane (arrows). G, H. Pseudoinclusions. I. Intranuclear tubular profiles. J. Intranuclear filaments. Bars, 1 µm.

Figure 3

Cytoplasmic pathological changes. A. Three myofibers show “splitting” (arrows). Bar, 10 µm. B. The cytoplasmic surfaces abutting the extracellular matrix of a split myofiber are smooth. C. In contrast, this myofiber of the wild‐type mouse shows indentations wherever its surface contacts extracellular matrix. Bars, 5 µm.

Figure 4

Subsarcolemmal myofibrillar loss. A. Myofibrils of the central myofiber do not reach the cell border. Arrows denote the border of the filamented cytoplasm. B. Electron micrograph of a parallel non‐serial section of the myofiber shown in A. One myonucleus in the myofibril‐free subsarcolemmal region shows clumped chromatin. Bars, 50 µm. C. Higher magnification of a subsarcolemmal area containing honeycomb structures, sarcotubular profiles, and polysomes. Bar, 5 µm.