A perinuclear actin cap regulates nuclear shape

Abstract

Defects in nuclear morphology often correlate with the onset of disease, including cancer, progeria, cardiomyopathy, and muscular dystrophy. However, the mechanism by which a cell controls its nuclear shape is unknown. Here, we use adhesive micropatterned surfaces to control the overall shape of fibroblasts and find that the shape of the nucleus is tightly regulated by the underlying cell adhesion geometry. We found that this regulation occurs through a dome-like actin cap that covers the top of the nucleus. This cap is composed of contractile actin filament bundles containing phosphorylated myosin, which form a highly organized, dynamic, and oriented structure in a wide variety of cells. The perinuclear actin cap is specifically disorganized or eliminated by inhibition of actomyosin contractility and rupture of the LINC complexes, which connect the nucleus to the actin cap. The organization of this actin cap and its nuclear shape-determining function are disrupted in cells from mouse models of accelerated aging (progeria) and muscular dystrophy with distorted nuclei caused by alterations of A-type lamins. These results highlight the interplay between cell shape, nuclear shape, and cell adhesion mediated by the perinuclear actin cap.LINC complexes ͉ nucleus I n 1921, Champy and Carleton suggested an apparent correlation between the shape of various types of animal cells and the shape of their respective nuclei (1). Moreover, defects in nuclear shape are routinely used in the lab and in clinical settings as markers of disease and differentiation in human cells and tissues (2). However, remarkably little is known about the factors that determine nuclear morphology in living cells. In particular, the molecular mechanisms that govern the shape of the interphase nucleus are unknown. Here we show that an actin filament structure that forms a cap or dome located above the apical surface of the nucleus tightly controls nuclear shape and identify key associated cytoskeletal regulators of its organization and nuclear shape-determining function. The organization and function of the perinuclear actin cap are deregulated in diseased cells with distorted nuclei. Results and DiscussionTo test the hypothesis of a correlation between the shape of the nucleus and the overall cell shape, mouse embryonic fibroblasts were dispersed on fibronectin (FN)-coated glass substrates. Using morphometric analysis, we found that nuclear shape and cellular shape correlated ( Fig. 1 A and B). Shape factor, defined as 4 A/P 2 (where A and P are the nuclear area and perimeter), approaches 1 for a rounded nucleus and approaches 0 for an elongated nucleus. Elongated cells typically showed an elongated nucleus of low shape factor; rounded cells showed a rounded nucleus of high shape factor (Fig. 1 A). To control cell shape and, therefore, be able to quantify nuclear shape as a function of cell shape, we developed adhesive FN-coated micropatterned stripes of width ranging between 10 and 50 m, which alternated with stripes covered with nonadhesive poly...

Spatial coordination between cell and nuclear shape within micropatterned endothelial cells

“…3a and Supplementary Movie 3). According to the observations of Khatau et al 18 , apical F-actin network in elongated cells was observed to become oriented with respect to the long cell axis ( Supplementary Fig. S3).…”

“…In addition, given recent interests on molecular linkages between apical actin filaments and nuclear membrane 18,19 , it will be interesting to investigate whether such links exist for central actin filaments and influence the force transmission process.…”

“…This assumption implies that the cytoskeleton applies forces to modulate nuclear shape and size in response to cell shape changes. Interestingly, Khatau et al 18 have reported the presence of a dome-like actin structure, called the actin cap, that covers the top of the nucleus, and their results suggest that this structure is involved in a part of the nuclear shape deformation. More recently, a physical connection between an array of actin filaments located above the nucleus and the nuclear membrane, called transmembrane actin-associated nuclear lines, have been observed to be implicated in the intracellular localization of the nucleus 19 .…”

Nanofiber scaffolds influence organelle structure and function in bone marrow stromal cells

“…[39][40][41] For mouse osteocytes, Himeno-Ando et al observed in situ a larger nucleus/cell volume ratio in tibial bones as compared to parietal bones, 42 which suggests that there may be different functional or microenvironmental demands placed on the osteoblasts in these bones. A number of mechanisms for how changes to cell and nuclear shape may lead to changes in cell function and gene expression are being advanced.…”

Nuclear deformability and telomere dynamics are regulated by cell geometric constraints

“…Recent studies have revealed that cytoskeletal organization and nuclear morphology are regulated by extracellular mechanical signals, such as substrate stiffness and geometry (32)(33)(34)(35)(36)(37)(38)(39)(40)(41). With the cytoskeleton physically linked to the nucleoskeleton, these extracellular mechanical signals can therefore be used to mediate changes in chromatin structure.…”