Karyopherin Alpha 1 Regulates Satellite Cell Proliferation and Survival by Modulating Nuclear Import

Abstract

Satellite cells are stem cells with an essential role in skeletal muscle repair. Precise regulation of gene expression is critical for proper satellite cell quiescence, proliferation, differentiation and self -renewal. Nuclear proteins required for gene expression are dependent on the nucleocytoplasmic transport machinery to access to nucleus, however little is known about regulation of nuclear transport in satellite cells. The best characterized nuclear import pathway is classical nuclear import which depends on a classical nuclear localization signal (cNLS) in a cargo protein and the heterodimeric import receptors, karyopherin alpha (KPNA) and beta (KPNB). Multiple KPNA1 paralogs exist and can differ in importing specific cNLS proteins required for cell differentiation and function. We show that transcripts for six Kpna paralogs underwent distinct changes in mouse satellite cells during muscle regeneration accompanied by changes in cNLS proteins in nuclei. Depletion of KPNA1, the most dramatically altered KPNA, caused satellite cells in uninjured muscle to prematurely activate, proliferate and undergo apoptosis leading to satellite cell exhaustion with age. Increased proliferation of satellite cells led to enhanced muscle regeneration at early stages of regeneration. In addition, we observed impaired nuclear localization of two key KPNA1 cargo proteins: p27, a cyclin-dependent kinase inhibitor associated with cell cycle control and lymphoid enhancer factor 1, a critical co-transcription factor for β-catenin. These results indicate that regulated nuclear import of proteins by KPNA1 is critical for satellite cell proliferation and survival and establish classical nuclear import as a novel regulatory mechanism for controlling satellite cell fate.

Importin α: functions as a nuclear transport factor and beyond

“… 81 ) Analysis of KPNA1 knockout mice further revealed that KPNA1 is important for muscle regeneration. 83 ) That is, KPNA1 knockout caused muscle satellite cells to prematurely activate and undergo apoptosis, which led to the exhaustion of muscle satellite cells. On the other hand, knocking out KPNA7 results in reduced reproductivity and fetal lethality in females.…”

A tale of two niches: differential functions for VCAM-1 in satellite cells under basal and injured conditions

“…Mononucleated cells were isolated from muscles and labeled with antibodies against Annexin V and PI to detect apoptotic cells (Fig. 3D), a method previously shown to correlate well with caspase 3 analyses of apoptotic SC in histologic sections (10). The percentage of early apoptotic (Annexin V ϩ /PI Ϫ ) tdTomato ϩ Vcam1 Ϫ/Ϫ SC was approximately twofold higher than Vcam1 ϩ/ϩ (Fig.…”

“…To analyze in vivo satellite cell apoptosis, isolated mononucleated cells from hindlimb muscle were labeled with propidium iodide (PI) and 1:40 Annexin V-FITC (Biolegend) in 10 mM HEPES, 140 mM NaCl, and 2.5 mM CaCl2 (pH 7.4) (10). Apoptotic cells were defined as Annexin V ϩ /PI Ϫ from the tdTomato ϩ satellite cell population (10).…”

Role of Nucleoporins and Transport Receptors in Cell Differentiation

“…Cells with depleted importin α5 exhibit premature activation and proliferation, leading to exhaustion and cell death. Moreover, critical protein cargoes are disrupted because of the impaired localization of importin α5 (Choo et al, 2016). FIGURE 2 | Model of nuclear import and export.…”