Keeping the LINC: the importance of nucleocytoskeletal coupling in intracellular force transmission and cellular function

Abstract

Providing a stable physical connection between the nucleus and the cytoskeleton is essential for a wide range of cellular functions and it could also participate in mechanosensing by transmitting intra- and extra-cellular mechanical stimuli via the cytoskeleton to the nucleus. Nesprins and SUN proteins, located at the nuclear envelope, form the LINC (linker of nucleoskeleton and cytoskeleton) complex that connects the nucleus to the cytoskeleton; underlying nuclear lamins contribute to anchoring LINC complex components at the nuclear envelope. Disruption of the LINC complex or loss of lamins can result in disturbed perinuclear actin and intermediate filament networks and causes severe functional defects, including impaired nuclear positioning, cell polarization and cell motility. Recent studies have identified the LINC complex as the major force-transmitting element at the nuclear envelope and suggest that many of the aforementioned defects can be attributed to disturbed force transmission between the nucleus and the cytoskeleton. Thus mutations in nesprins, SUN proteins or lamins, which have been linked to muscular dystrophies and cardiomyopathies, may weaken or completely eliminate LINC complex function at the nuclear envelope and result in impaired intracellular force transmission, thereby disrupting critical cellular functions.

Figure 1

Impaired cell polarization in mouse embryonic fibroblasts stably expressing dominant negative nesprin constructs. Cells were plated on fibronectin-micropatterned substrates (red) and fluorescently stained for γ-tubulin (green) as centrosomal marker and Hoechst 33342 nuclear stain (blue). (A) Control fibroblast expressing mCherry only polarize towards the bowed front, characterized by a rearward nuclear position and a forward facing centrosome position (arrow). (B) Cells expressing a dominant negative nesprin mCherry fusion construct fail to polarize properly. This figure was originally published in [29]: Lombardi et al. The Interaction between Nesprins and Sun Proteins at the Nuclear Envelope Is Critical for Force Transmission between the Nucleus and Cytoskeleton. J Biol Chem. 2011. 286: 26743-26753 © the American Society for Biochemistry and Molecular Biology.

Figure 2

Examples of nucleo-cytoskeletal force transmission and relevance in cellular function. Forces acting on the nucleus are depicted as small black arrows (A) Rearward nuclear movement in polarizing cells via retrograde flow of actin cables coupled to the nucleus trough nesprin-2/SUN2 [23]. (B) Interkinetic nuclear migration in neurons. Microtubule-associated motors move the nucleus during the cell cycle. Cell cycle phases are indicated as G1, S, G2, and M. (C) Nuclear movement in a migrating cell passing through a narrow constriction. (D) Nuclear anchoring at neuromuscular junctions via lamins, nesprins, and SUN proteins.

Figure 3

Representative displacement vector map of induced cytoskeletal deformations in a microneedle manipulation assay. Precisely controlled localized strain was applied with a microneedle near the nucleus towards the cell periphery (yellow arrow). Final cytoskeletal displacements (red vectors) were computed by tracking fluorescently labeled mitochondria (Mitotracker Green) during force application. The fibroblast nucleus was labeled with Hoechst 33342 nuclear stain (blue). Shown are the final displacements at the end of the localized strain application. Vector length shown at 2× magnification for clarity.