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

Abstract

The complex comprising serum response factor (SRF) and megakaryoblastic leukemia 1 protein (Mkl1) promotes myofibroblast differentiation during wound healing. SRF−Mkl1 is sensitive to the mechanical properties of the extracellular environment; but how cells sense and transduce mechanical cues to modulate SRF−Mkl1-dependent gene expression is not well understood. Here, we demonstrate that the nuclear lamina-associated inner nuclear membrane protein Emerin stimulates SRF−Mkl1-dependent gene activity in a substrate stiffness-dependent manner. Specifically, Emerin was required for Mkl1 nuclear accumulation and maximal SRF−Mkl1-dependent gene expression in response to serum stimulation of cells grown on stiff substrates but was dispensable on more compliant substrates. Focal adhesion area was also reduced in cells lacking Emerin, consistent with a role for Emerin in sensing substrate stiffness. Expression of a constitutively active form of Mkl1 bypassed the requirement for Emerin in SRF−Mkl1-dependent gene expression and reversed the focal adhesion defects evident in Emd KO fibroblasts. Together, these data indicate that Emerin, a conserved nuclear lamina protein, couples extracellular matrix mechanics and SRF−Mkl1-dependent transcription.

Lamin A/C Deficiency Enables Increased Myosin2 Bipolar Filament Ensembles Which Promote Divergent Actomyosin Network Anomalies Through Self Organization

“…Our results ( Fig. 5A) and those of others (Willer and Carroll, 2017), nonetheless, suggest complex and differential regulation of MKL1/SRF gene targets in LMNA deficient cells, as some genes such as VCL are repressed at steady state and not activated. Repression of some SRF targets like VCL in the face of increased MKL signaling may involve other gene specific regulatory factors.…”

“…Ectopic expression of two different constitutively active MKL1 mutant proteins (Flag-MKL1-N100, lacks an N-terminal actin binding motif required for its Gactin dependent cytoplasmic retention; and Flag-MKL1-STS/A, S449, S454, and T450 mutated to A, is refractory to ERK regulated nuclear export; Muehlich et al, 2008), showed both nuclear and cytoplasmic localization ( Fig. S3), as observed by others (Willer and Carroll, 2017).…”

“…The aforementioned evidence suggesting increased MKL1 activation in LMNA deficient cells is not necessarily in conflict with an initial report by Ho et al (2013b), which advocated abrogated MKL1 signaling. Combined results from other reports suggest impairment of mechanosensitive transcription factor signaling in LINC module disrupted cells may be contextual to factors such as cell and extracellular matrix mechanical properties (Bertrand et al, 2014;Willer and Carroll, 2017). For example, impaired MKL1 and YAP nuclear translocation may be anticipated in mechanical contexts that favor steady states of self-organized actomyosin network disassembly.…”

Mass spectrometry‐based proteomic analysis of FSCN1‐interacting proteins in laryngeal squamous cell carcinoma cells

“…ARPC3, a member of human Arp2/3 protein complex, functions in regulating tight junctions . EMD, a serine‐rich nuclear membrane protein, functions in regulating focal adhesion . The interactions between FSCN1 and these cell–cell adhesion‐associated proteins further strengthen the roles of FSCN1 in cell adhesion.…”

Nuclear Lamins and Emerin Are Differentially Expressed in Osteosarcoma Cells and Scale with Tumor Aggressiveness

“…In mammals, the LMNA gene gives rise to lamins A and C, but also minor isoforms as lamin C2 and Adelta10, by alternative RNA splicing [3], whereas B-type lamins are encoded by the LMNB1 (lamins B1) and LMNB2 (lamins B2 and B3) genes [4][5][6]. B-type lamins are ubiquitously expressed and considered essential for cell survival, whereas A-type lamins change during development and cell differentiation stages, being response to serum stimulation of cells grown on stiff substrates but was dispensable on more compliant substrates [41].…”