Teresa Sullivan

Lamin A/C deficiency causes defective nuclear mechanics and mechanotransduction

Mutations in the lamin A/C gene (LMNA) cause a variety of human diseases including Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy, and Hutchinson-Gilford progeria syndrome. The tissue-specific effects of lamin mutations are unclear, in part because the function of lamin A/C is incompletely defined, but the many muscle-specific phenotypes suggest that defective lamin A/C could increase cellular mechanical sensitivity. To investigate the role of lamin A/C in mechanotransduction, we subjected lamin A/C-deficient mouse embryo fibroblasts to mechanical strain and measured nuclear mechanical properties and strain-induced signaling. We found that Lmna-/- cells have increased nuclear deformation, defective mechanotransduction, and impaired viability under mechanical strain. NF-kappaB-regulated transcription in response to mechanical or cytokine stimulation was attenuated in Lmna-/- cells despite increased transcription factor binding. Lamin A/C deficiency is thus associated with both defective nuclear mechanics and impaired mechanically activated gene transcription. These findings suggest that the tissue-specific effects of lamin A/C mutations observed in the laminopathies may arise from varying degrees of impaired nuclear mechanics and transcriptional activation.

A progeroid syndrome in mice is caused by defects in A-type lamins.

Numerous studies of the underlying causes of ageing have been attempted by examining diseases associated with premature ageing, such as Werners syndrome and Hutchinson–Gilford progeria syndrome (HGPS). HGPS is a rare genetic disorder resulting in phenotypes suggestive of accelerated ageing, including shortened stature, craniofacial disproportion, very thin skin, alopecia and osteoporosis, with death in the early teens predominantly due to atherosclerosis. However, recent reports suggest that developmental abnormalities may also be important in HGPS. Here we describe the derivation of mice carrying an autosomal recessive mutation in the lamin A gene (Lmna) encoding A-type lamins, major components of the nuclear lamina. Homozygous mice display defects consistent with HGPS, including a marked reduction in growth rate and death by 4 weeks of age. Pathologies in bone, muscle and skin are also consistent with progeria. The Lmna mutation resulted in nuclear morphology defects and decreased lifespan of homozygous fibroblasts, suggesting premature cell death. Here we present a mouse model for progeria that may elucidate mechanisms of ageing and development in certain tissue types, especially those developing from the mesenchymal cell lineage.

Novel roles for A-type lamins in telomere biology and the DNA damage response pathway

A‐type lamins are intermediate filament proteins that provide a scaffold for protein complexes regulating nuclear structure and function. Mutations in the LMNA gene are linked to a variety of degenerative disorders termed laminopathies, whereas changes in the expression of lamins are associated with tumourigenesis. The molecular pathways affected by alterations of A‐type lamins and how they contribute to disease are poorly understood. Here, we show that A‐type lamins have a key role in the maintenance of telomere structure, length and function, and in the stabilization of 53BP1, a component of the DNA damage response (DDR) pathway. Loss of A‐type lamins alters the nuclear distribution of telomeres and results in telomere shortening, defects in telomeric heterochromatin, and increased genomic instability. In addition, A‐type lamins are necessary for the processing of dysfunctional telomeres by non‐homologous end joining, putatively through stabilization of 53BP1. This study shows new functions for A‐type lamins in the maintenance of genomic integrity, and suggests that alterations of telomere biology and defects in DDR contribute to the pathogenesis of lamin‐related diseases.

Actin-myosin–based contraction is responsible for apoptotic nuclear disintegration

Membrane blebbing during the apoptotic execution phase results from caspase-mediated cleavage and activation of ROCK I. Here, we show that ROCK activity, myosin light chain (MLC) phosphorylation, MLC ATPase activity, and an intact actin cytoskeleton, but not microtubular cytoskeleton, are required for disruption of nuclear integrity during apoptosis. Inhibition of ROCK or MLC ATPase activity, which protect apoptotic nuclear integrity, does not affect caspase-mediated degradation of nuclear proteins such as lamins A, B1, or C. The conditional activation of ROCK I was sufficient to tear apart nuclei in lamin A/C null fibroblasts, but not in wild-type fibroblasts. Thus, apoptotic nuclear disintegration requires actin-myosin contractile force and lamin proteolysis, making apoptosis analogous to, but distinct from, mitosis where nuclear disintegration results from microtubule-based forces and from lamin phosphorylation and depolymerization.

Myonuclear Degeneration in LMNA Null Mice

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.