The LIM domain protein nTRIP6 modulates the dynamics of myogenic differentiation

Abstract

The process of myogenesis which operates during skeletal muscle regeneration involves the activation of muscle stem cells, the so-called satellite cells. These then give rise to proliferating progenitors, the myoblasts which subsequently exit the cell cycle and differentiate into committed precursors, the myocytes. Ultimately, the fusion of myocytes leads to myofiber formation. Here we reveal a role for the transcriptional co-regulator nTRIP6, the nuclear isoform of the LIM-domain protein TRIP6, in the temporal control of myogenesis. In an in vitro model of myogenesis, the expression of nTRIP6 is transiently up-regulated at the transition between proliferation and differentiation, whereas that of the cytosolic isoform TRIP6 is not altered. Selectively blocking nTRIP6 function results in accelerated early differentiation followed by deregulated late differentiation and fusion. Thus, the transient increase in nTRIP6 expression appears to prevent premature differentiation. Accordingly, knocking out the Trip6 gene in satellite cells leads to deregulated skeletal muscle regeneration dynamics in the mouse. Thus, dynamic changes in nTRIP6 expression contributes to the temporal control of myogenesis.

Short internal open reading frames regulate the translation of N-terminally truncated proteoforms

“…Thus, the transient increase in nTRIP6 expression occurs via increased translation initiation at AUG2. We have previously shown that nTRIP6 represses premature myoblast differentiation, allowing proper myocyte differentiation and fusion at later stages (Norizadeh Abbariki et al, 2021). Accordingly, treatment of C2C12 myoblasts with the AUG2 PNA accelerated the expression of Myog mRNA, used as an indicator of early myocytic differentiation (Edmondson & Olson, 1989) and delayed the expression of Tnni2, a late differentiation gene (Lin et al, 1994), as compared to untreated cells or cells treated with the control PNA (Fig 1F,G).…”

“…Our previous observation that the levels of nTRIP6 transiently increased during C2C12 myoblast differentiation whereas those of TRIP6 did not vary (Norizadeh Abbariki et al, 2021) suggests an increased translation at AUG2. To address this question, we used the AUG2 and the control PNAs fused to a cell-penetrating peptide 15,16 in a C2C12 differentiation assay.…”

“…Interestingly, mTORC1 has been reported to inhibit differentiation at early stages (Ge et al, 2011;Wilson et al, 2016). Given that nTRIP6 prevents premature differentiation (Norizadeh Abbariki et al, 2021) (Fig 1), it is tempting to speculate that mTORC1 inhibits early differentiation at least in part via the up-regulation of nTRIP6 translation.…”

“…This repression is required for the subsequent late differentiation and fusion of myocytes. Furthermore, the expression of nTRIP6 is transiently upregulated at the transition between proliferation and differentiation, while that of TRIP6 is not (Norizadeh Abbariki et al, 2021).…”

“…In the case of TRIP6, this nuclear function is exerted by a shorter, exclusively nuclear proteoform, which we have termed nTRIP6 (Kassel et al, 2004;Diefenbacher et al, 2008Diefenbacher et al, , 2010Kemler et al, 2016). We have recently reported a role for nTRIP6 in the regulation of myogenesis (Norizadeh Abbariki et al, 2021). This process relies on resident adult stem cells, the so-called satellite cells (Schmidt et al, 2019).…”