The Differential Formation of the LINC-Mediated Perinuclear Actin Cap in Pluripotent and Somatic Cells

Shyam B Khatau,Denis Wirtz,Sravanti Kusuma,Linzhao Cheng,Sharon Gerecht,Prashant Mali,Donny Hanjaya-Putra,Jerry S H Lee,Laurent Kreplak

doi:10.1371/journal.pone.0036689

Abstract

The actin filament cytoskeleton mediates cell motility and adhesion in somatic cells. However, whether the function and organization of the actin network are fundamentally different in pluripotent stem cells is unknown. Here we show that while conventional actin stress fibers at the basal surface of cells are present before and after onset of differentiation of mouse (mESCs) and human embryonic stem cells (hESCs), actin stress fibers of the actin cap, which wrap around the nucleus, are completely absent from undifferentiated mESCs and hESCs and their formation strongly correlates with differentiation. Similarly, the perinuclear actin cap is absent from human induced pluripotent stem cells (hiPSCs), while it is organized in the parental lung fibroblasts from which these hiPSCs are derived and in a wide range of human somatic cells, including lung, embryonic, and foreskin fibroblasts and endothelial cells. During differentiation, the formation of the actin cap follows the expression and proper localization of nuclear lamin A/C and associated linkers of nucleus and cytoskeleton (LINC) complexes at the nuclear envelope, which physically couple the actin cap to the apical surface of the nucleus. The differentiation of hESCs is accompanied by the progressive formation of a perinuclear actin cap while induced pluripotency is accompanied by the specific elimination of the actin cap, and that, through lamin A/C and LINC complexes, this actin cap is involved in progressively shaping the nucleus of hESCs undergoing differentiation. While, the localization of lamin A/C at the nuclear envelope is required for perinuclear actin cap formation, it is not sufficient to control nuclear shape.

Highlights

The intermediate filament type A lamins, but not type B lamins, are absent from the nuclear lamina in undifferentiated stem cells [1,2,3]
The results in this study suggest that the linkers of nucleus and cytoskeleton (LINC) complexes and the actin cap are intimately involved in stem cell differentiation and that their presence and proper localization at the nuclear envelope are likely required for normal development
While Nesprin 2 and Nesprin 3 appear to be expressed in undifferentiated mouse and human pluripotent cells, they do not localize to the nuclear envelope until lamin A/C is expressed, after the induction of differentiation

Summary

Introduction

The intermediate filament type A lamins, but not type B lamins, are absent from the nuclear lamina in undifferentiated stem cells [1,2,3]. Differentiated and undifferentiated cells show strikingly different nuclear shape, plasticity, and mechanical stiffness [2,4,5], suggesting that lamin A/C may play a critical role in preventing stem cells from shaping their nucleus. The actin cap, not conventional basal and cortical actin stress fibers, is absent from cells deficient in lamin A/C, a phenotype recapitulated in cells where LINC complexes are displaced from the nuclear envelope to the ER and cytoplasm [5,13]. Whether undifferentiated stem cells, which lack lamin A/C, feature an actin cap and whether the actin cap contributes to nuclear shaping in stem cells undergoing differentiation are unknown

Methods

Results

Conclusion