Opposing roles for distinct LINC complexes in regulation of the small GTPase RhoA

Abstract

Linker of Nucleoskeleton and Cytoskeleton (LINC) complexes span the nuclear envelope and transduce force from dynamic cytoskeletal networks to the nuclear lamina. Here we show that LINC complexes also signal from the nuclear envelope to critical regulators of the actin cytoskeleton. Specifically, we find that LINC complexes that contain the inner nuclear membrane protein Sun2 promote focal adhesion assembly by activating the small GTPase RhoA. A key effector in this process is the transcription factor/coactivator complex composed of SRF/Mkl1. A constitutively active form of SRF/Mkl1 was not sufficient to induce focal adhesion assembly in cells lacking Sun2, however, suggesting that LINC complexes support RhoA activity through a transcription-independent mechanism. Strikingly, we also find that the inner nuclear membrane protein Sun1 antagonizes Sun2 LINC complexes and inhibits RhoA activation and focal adhesion assembly. Thus different LINC complexes have opposing roles in the transcription-independent control of the actin cytoskeleton through the small GTPase RhoA.

Substrate stiffness-dependent regulation of the SRF−Mkl1 co-activator complex requires the inner nuclear membrane protein Emerin

“…Equally important is to discover how the Emerin-independent functions of Lamins A/C promote Mkl1 nuclear localization in cells grown on stiff substrates. We have demonstrated recently that nuclear envelopespanning linker of nucleoskeleton and cytoskeleton (LINC) complexes, which couple F-actin, microtubules and intermediate filaments to the nuclear lamina, promote focal adhesion assembly through a transcription-independent mechanism (Thakar et al, 2017). Thus, Lamins A/C could enhance Mkl1 nuclear accumulation through LINC complexes in Emd KO cells.…”

“…Emerin localization to the INM or ONM was quantified by measuring the intensity levels of Emerin fluorescence at the nuclear envelope in Triton X-100 or digitoninpermeabilized cells, respectively (Le et al, 2016). Focal adhesions were quantified as described (Thakar et al, 2017). The area of all FAs from each cell was summed and then divided by the cell area to determine total FA area per cell and the percent of vinculin-positive area per cell, respectively.…”

SUN1/2 Are Essential for RhoA/ROCK-Regulated Actomyosin Activity in Isolated Vascular Smooth Muscle Cells

“…Feedback also exists between the NE and matrix adhesions and disruption of lamins A/C function or overexpression of the dominant negative nesprin KASH-domain triggers focal adhesion reorganisation and changes in cell motility [18][19][20]. The nature of this feedback remains unknown, however, lamin A/C and SUN2 have recently been demonstrated to regulate Rac1 and RhoA activity, respectively [20,21]. Although in some contexts LINC complex components are partially redundant, several lines of data suggest that the LINC complex is tailored for cell-type-specific functions.…”

Ablation of SUN2-containing LINC complexes drives cardiac hypertrophy without interstitial fibrosis

“…How could SUN2 LINC complexes influence sarcomere contractile function? Beyond its direct interactions with the actin cytoskeleton, the LINC complex is implicated as a regulator of RhoA activity (Thakar et al, 2017), interacts with other modulators of actin organization and function, such as the formin FHOD1 in fibroblasts (Kutscheidt et al, 2014), and is known to influence actin dynamics (Lammerding et al, 2004;Lee et al, 2007;Hale et al, 2008;Stewart-Hutchinson et al, 2008;Khatau et al, 2009;Luxton et al, 2010b;Folker et al, 2011;Kim et al, 2012). However, it should be noted that while increased cardiac contractility at the organ-level is frequently observed in HCM, cell autonomous hypercontractility is not always observed in these disease models (Moore et al, 2012).…”