Nuclear Constriction Segregates Mobile Nuclear Proteins Away from Chromatin

Abstract

As a cancer cell squeezes through adjacent tissue, penetrates a basement membrane, or enters the smallest blood capillaries, its nucleus can be highly constricted, but any effects on chromatin density and other nuclear factors are poorly understood. Here, in cancer cell migration through rigid micropores and also in passive pulling into micropipettes, local compaction of chromatin is observed coincident in space and time with depletion of mobile factors. Hetero/eu-chromatin has been previously estimated from molecular mobility measurements to occupy a volume fraction f roughly two-thirds of the nucleus, but based on the relative intensity of DNA and histones in several cancer cell lines drawn into narrow constrictions, f can easily increase locally to nearly 100%. By contrast, mobile proteins in the nucleus, including a dozen that function as DNA repair proteins (e.g. 53BP1) or nucleases (e.g. Cas9, FokI), are seen to exhibit a much reduced density within the constriction, approaching 0%. Such loss of mobile nuclear factors—compounded by the occasional rupture of the nuclear envelope—has important functional consequences for the cell. Constricted migration indeed delays DNA cleavage by a FokI-lacR fusion of a target locus integrated into chromosome 1.