Physical plasticity of the nucleus in stem cell differentiation

doi:10.1073/pnas.0702576104

Physical plasticity of the nucleus in stem cell differentiation

*Molecular and Cell Biophysics Laboratory, 129 Towne Building, University of Pennsylvania, Philadelphia, PA 19104;
^†Departments of Chemical and Biomedical Engineering, 5000 Forbes Avenue, Carnegie Mellon University, Pittsburgh, PA 15213; and
^‡Division of Developmental and Regenerative Medicine, University of Pittsburgh, Pittsburgh, PA 15213

Edited by L. B. Freund, Brown University, Providence, RI, and approved August 10, 2007 (received for review March 19, 2007)

Abstract

Cell differentiation in embryogenesis involves extensive changes in gene expression structural reorganization within the nucleus, including chromatin condensation and nucleoprotein immobilization. We hypothesized that nuclei in naive stem cells would therefore prove to be physically plastic and also more pliable than nuclei in differentiated cells. Micromanipulation methods indeed show that nuclei in human embryonic stem cells are highly deformable and stiffen 6-fold through terminal differentiation, and that nuclei in human adult stem cells possess an intermediate stiffness and deform irreversibly. Because the nucleo-skeletal component Lamin A/C is not expressed in either type of stem cell, we knocked down Lamin A/C in human epithelial cells and measured a deformability similar to that of adult hematopoietic stem cells. Rheologically, lamin-deficient states prove to be the most fluid-like, especially within the first ≈10 sec of deformation. Nuclear distortions that persist longer than this are irreversible, and fluorescence-imaged microdeformation with photobleaching confirms that chromatin indeed flows, distends, and reorganizes while the lamina stretches. The rheological character of the nucleus is thus set largely by nucleoplasm/chromatin, whereas the extent of deformation is modulated by the lamina.

Footnotes

^§To whom correspondence should be addressed. E-mail: discher{at}seas.upenn.edu
Author contributions: J.D.P., K.N.D., P.J.S., and D.E.D. designed research; J.D.P., K.N.D., and F.L.Z. performed research; P.J.S. contributed new reagents/analytic tools; J.D.P., K.N.D., P.J.S., and D.E.D. analyzed data; and J.D.P., K.N.D., P.J.S., and D.E.D. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0702576104/DC1.
Abbreviations:

ESC,

embryonic stem cell;

HSC,

hematopoietic stem cell;

shRNA,

small hairpin RNA.