Loss of emerin at the nuclear envelope disrupts the Rb1/E2F and MyoD pathways during muscle regeneration

Abstract

Emery-Dreifuss muscular dystrophy (EDMD1) is caused by mutations in either the X-linked gene emerin (EMD) or the autosomal lamin A/C (LMNA) gene. Here, we describe the derivation of mice lacking emerin in an attempt to derive a mouse model for EDMD1. Although mice lacking emerin show no overt pathology, muscle regeneration in these mice revealed defects. A bioinformatic array analysis of regenerating Emd null muscle revealed abnormalities in cell-cycle parameters and delayed myogenic differentiation, which were associated with perturbations to transcriptional pathways regulated by the retinoblastoma (Rb1) and MyoD genes. Temporal activation of MyoD transcriptional targets was significantly delayed, whereas targets of the Rb1/E2F transcriptional repressor complex remained inappropriately active. The inappropriate modulation of Rb1/MyoD transcriptional targets was associated with up-regulation of Rb1, MyoD and their co-activators/repressors transcripts, suggesting a compensatory effort to overcome a molecular block to differentiation at the myoblast/myotube transition during regeneration. This compensation appeared to be effective for MyoD transcriptional targets, although was less effective for Rb1 targets. Analysis of Rb1 phosphorylation states showed prolonged hyper-phosphorylation at key developmental stages in Emd null myogenic cells, both in vivo and in vitro. We also analyzed the same pathways in Lmna null muscle, which shows extensive dystrophy. Surprisingly, Lmna null muscle did not show the same perturbations to Rb- and MyoD-dependent pathways. We did observe increased transcriptional expression of Lap2alpha and delayed expression of Rb1, which may regulate alternative transcriptional pathways in the Lmna null myoblasts. We suggest that the dominant LMNA mutations seen in many clinically disparate laminopathies may similarly alter Rb function, with regard to either the timing of exit from the cell cycle or terminal differentiation programs or both.

Mutation in TOR1AIP1 encoding LAP1B in a form of muscular dystrophy: A novel gene related to nuclear envelopathies

“…It is known that lamin A/C and emerin are involved in muscle transcriptional pathways [33][34][35][36][37]. One study proposed that LAP1 might bind indirectly to mitotic chromosomes via a linker protein [9].…”

TDP-43 regulates retinoblastoma protein phosphorylation through the repression of cyclin-dependent kinase 6 expression

“…Further investigation will be required to understand the onset of nuclear membrane instability upon loss of TDP-43. Interestingly, recent work showed that nuclear membrane dysfunction, particularly loss of emerin, is tied to misregulation of pRb pathway components (40,41) suggesting that nuclear membrane abnormalities caused by TDP-43 loss are mediated by pRb. These conclusions are supported by the lack of apoptosis and nuclear membrane disruption after TDP-43 silencing in Saos-2 cells that lack a functional pRb pathway.…”

Mouse models of the laminopathies

“…The in vivo regeneration of cardiotoxin treated Lmna −/− muscles appears to proceed normally and primary Lmna −/− myoblast cultures do not show any impairment in their differentiation to myotubes [21]. However immortalized Lmna −/− myoblasts are impaired in their differentiation to myotubes, [22], suggesting that the process of immortalization affects the myoblasts dependency on having a functional lamina for differentiation to proceed.…”

“…Temporal activation of MyoD transcriptional targets was significantly delayed, whereas targets of the Rb1/E2F transcriptional repressor complex remained inappropriately active resulting in a delay in myoblast/myotube transition during regeneration. Cultured primary myoblasts from Emd null mice also showed a delay in fusing to form myotubes, which correlated with prolonged Rb1 hyperphosphorylation [21]. Despite these anomalies, mice (at least 1 year old) lacking emerin do not develop muscular dystrophy or cardiac conduction defects, suggesting that, in mice, loss of emerin may have few pathological consequences [21,23].…”