Nuclear Shape Changes Are Induced by Knockdown of the SWI/SNF ATPase BRG1 and Are Independent of Cytoskeletal Connections

Abstract

Changes in nuclear morphology occur during normal development and have been observed during the progression of several diseases. The shape of a nucleus is governed by the balance of forces exerted by nuclear-cytoskeletal contacts and internal forces created by the structure of the chromatin and nuclear envelope. However, factors that regulate the balance of these forces and determine nuclear shape are poorly understood. The SWI/SNF chromatin remodeling enzyme ATPase, BRG1, has been shown to contribute to the regulation of overall cell size and shape. Here we document that immortalized mammary epithelial cells show BRG1-dependent nuclear shape changes. Specifically, knockdown of BRG1 induced grooves in the nuclear periphery that could be documented by cytological and ultrastructural methods. To test the hypothesis that the observed changes in nuclear morphology resulted from altered tension exerted by the cytoskeleton, we disrupted the major cytoskeletal networks and quantified the frequency of BRG1-dependent changes in nuclear morphology. The results demonstrated that disruption of cytoskeletal networks did not change the frequency of BRG1-induced nuclear shape changes. These findings suggest that BRG1 mediates control of nuclear shape by internal nuclear mechanisms that likely control chromatin dynamics.

SMARCA4 regulates gene expression and higher-order chromatin structure in proliferating mammary epithelial cells

“…However, changes in nuclear shape can be induced from either external forces exerted by the cytoskeleton or via internal nuclear forces. Previous work showed that SMARCA4 knockdown results in nuclear shape alterations in MCF-10A cells (Imbalzano et al 2013b). However, the disruption of the cytoplasmic filaments (actin, tubulin, and cytokeratins) did not alter SMARCA4-dependent structural changes observed in the MCF-10A cells (Imbalzano et al 2013b).…”

“…Previous work showed that SMARCA4 knockdown results in nuclear shape alterations in MCF-10A cells (Imbalzano et al 2013b). However, the disruption of the cytoplasmic filaments (actin, tubulin, and cytokeratins) did not alter SMARCA4-dependent structural changes observed in the MCF-10A cells (Imbalzano et al 2013b). This implies that SMARCA4, apart from its chromatin remodeling function, might have additional roles in maintaining the structural integrity of the nucleus (Imbalzano et al 2013a).…”

“…Furthermore, previous work classifying genome-wide interactions according to their histone modifications and transcription factor binding revealed SMARCA4 enrichment at open chromatin regions, indicating a possible structural role for SMARCA4 (Lan et al 2012). In addition, SMARCA4 knockdown affects nuclear size (Hill et al 2004) and the integrity of nuclear shape via a mechanism independent of cytoskeletal connections (Imbalzano et al 2013b). Recently, SMARCA4 was shown to be involved in the lncRNA-dependent assembly of nuclear bodies (Kawaguchi et al 2015).…”

Sub-cellular force microscopy in single normal and cancer cells

“…The results of previous studies show there is a continuum of biological elements which establish a mechano-transduction cascade linking membrane displacements to cytoskeletal filaments, the nucleus and sub-nuclear structures, and cell integrin-substrate forces [27,28]. Thus, the biomechanical measurements cannot be assigned to a particular sub-structure, but dominated by the response of a sub-cellular structural domain.…”

Loss of the integral nuclear envelope protein SUN1 induces alteration of nucleoli

“…Second, Pap is a multichromatic cell stain, and the combination of colors highlights many different features of cellular structures, including chromatin condensation and cytokeratin expression. [32][33][34] In Pap-or DAPI-stained images, aberrant nuclear and chromatin structures were detected in lamin A/C knockdown cells (Fig. 1A and B).…”