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Andrew Stephens, Northwestern U., Separating the role of chromatin from lamins in mechanics and morphology of the cell nucleus

Date: Thu. February 22nd, 2018, 4:30 pm-5:30 pm
Location: Rockefeller 221 (Les Foldy Room)

Separating the role of chromatin from lamins in mechanics and morphology of the cell nucleus

Andrew Stephens, Northwestern U.

The nucleus is the 10 µm ellipse compartment in the cell which must properly transduce or resist biophysical
forces to dictate the spatial organization of the 2 meters of genome inside of it. Organization and
mechanotransduction determine the expression profile of genome which dictates cell function. Previous studies
revealed that the two major contributors to nuclear mechanics are lamins, protein intermediate filaments lining
the inner nuclear envelope, and chromatin, the DNA genome and its associated proteins, which fills the
nucleus. Alterations of lamins and chromatin compaction occur in many major human diseases including heart
disease, aging, muscular dystrophy and many cancers. In these disease nuclear mechanics, chromatin
organization and nuclear morphology alterations present as deformation or protrusions of the nucleus termed
blebs. Doctors have been using abnormal nuclear morphology for diagnosis and prognosis of cancer for nearly
80 years. However, the physical basis underlying these abnormal deformations remains unknown. To answer
this question, I developed a novel nuclear force measurement technique to investigate the relative
contributions of these two main nuclear components, chromatin and lamins. Using micropipettes we isolate a
nucleus from a living cell by gentle lysis of the cell membrane and attachment of micropipettes at opposite
ends of the nucleus. The “pull” pipette extends the nucleus while deflection of the “force” pipette, with a pre-
measured spring constant, provides a measurement of force. This novel approach revealed that chromatin and
its histone-mediated compaction state dictated initial force response (< 30% strain) while also contributing as a
secondary factor to the lamin A dictated strain stiffening at longer extensions. Furthermore, chromatin’s
histone-modification- based nuclear rigidity impacts nuclear morphology, in that weakening causes nuclear
blebbing while strengthening rescues nuclear shape in model and patient cells of aging disease progeria.
Thus, chromatin is a major mechanical component of the nucleus that dictates nuclear morphology relevant to
both healthy cells and disease.


host: Michael Hinczewski







host: Michael Hinczewski

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