Nuclear mechanotransduction: sensing the force from within

Available online at www.sciencedirect.com

ScienceDirect Nuclear mechanotransduction: sensing the force from within Avathamsa Athirasala1, Nivi Hirsch2 and Amnon Buxboim1,2 The cell nucleus is a hallmark of eukaryotic evolution, where gene expression is regulated and the genome is replicated and repaired. Yet, in addition to complex molecular processes, the nucleus has also evolved to serve physical tasks that utilize its optical and mechanical properties. Nuclear mechanotransduction of externally applied forces and extracellular stiffness is facilitated by the physical connectivity of the extracellular environment, the cytoskeleton and the nucleoskeletal matrix of lamins and chromatin. Nuclear mechanosensor elements convert applied tension into biochemical cues that activate downstream signal transduction pathways. Mechanoregulatory networks stabilize a contractile cell state with feedback to matrix, cell adhesions and cytoskeletal elements. Recent advances have thus provided mechanistic insights into how forces are sensed from within, that is, in the nucleus where cell-fate decision-making is performed. Addresses 1 Alexander Grass Center for Bioengineering, School of Computer Science and Engineering, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel 2 Institute of Life Sciences, The Hebrew University of Jerusalem, Jerusalem 9190401, Israel Corresponding author: Buxboim, Amnon ([email protected])

Current Opinion in Cell Biology 2017, 46:119–127 This review comes from a themed issue on Cell nucleus Edited by Roland Foisner and Aaron F Straight

http://dx.doi.org/10.1016/j.ceb.2017.04.004

embryo development [8,9]. Recent advancements have provided mechanistic insights of how mechanical forces are converted into biochemical cues inside the cell nucleus and how the emerging signals propagate via conserved pathways to regulate gene expression and define a contractile cell state. Here we review the recent advancements in our understanding of the structural, physical and molecular properties that facilitate nuclear mechanotransduction and discuss design principles of how cells sense forces from within.

Structural organization of nuclear lamina Resolving the structural organization of the nuclear lamina (NL) has been a long-standing experimental challenge [10,11]. The nuclear envelope (NE) is a highly crowded environment that is occupied by cytoplasmic intermediate filaments (IFs) and other perinuclear cytoskeletal arrangements on the cytoplasmic side, and dense formations of heterochromatin, DNA, nuclear lamins and lamin-binding proteins on the nucleoplasmic side [12,13]. Protein specificity obtained by high-resolution fluorescence microscopy displayed juxtaposed networks comprising either A- or B-type lamins in mammalian nuclei that cross-interact and are involved in chromatin maintenance and transcription regulation [14,15]. The remarkable recent advancements in cryo-electron tomography (cryo-TM) techniques have provided unparalleled resolution of the nucleus [16]. Nuclear lamins detected in Hela cells and in mouse embryonic fibroblasts (MEFs) were found to form thinner than expected filaments, 3.5–4 nm, with a persistence length > 500 nm [17,18]. In fact, lamins assemble into the thinnest cyto/nucleoskeletal filaments identified to date, even thinner than the actin ‘microfilament’.

0955-0674/ã 2017 Elsevier Ltd. All rights reserved.

Introduction Mechanosensing of externally applied forces and of extracellular stiffness is a fundamental property of solid tissue adhering cells [1,2]. It provides adhesive cells with a means to locomote towards stiff scar-like regions [3], modulate cancer cell malignancy [4], direct differentiation of adult stem and progenitor cells towards soft or stiff tissue lineages [5], regulate the exit from pluripotency of embryonic stem cells [6,7], and direct the very first differentiation process in mammalian preimplantation www.sciencedirect.com

The identity of lamin filaments was confirmed and a detailed description of their molecular scale organization was achieved using cryo-TM imaging of vimentin-null MEFs [18]. A- and B-type lamins were laterally decorated by an even longitudinal distribution of Ig-domains. In vitro assembly of A- and B-type paracrystals confirmed that lamin dimers polymerize head-to-tail via short overlapping tetrameric edge regions of 6 nm. Two halfstaggered lamin polymers assemble laterally to form a rod with alternating tetrameric-hexameric regions and 20 nm separation between two adjacent globular domain pairs. The spatial resolution (20 A˚) is insufficient to determine the polarity of the tetrameric protofilaments. Lamin assembly in Caenorhabditis elegans supports the formation of apolar filaments [19], yet the structural Current Opinion in Cell Biology 2017, 46:119–127

120 Cell nucleus

analysis of mammalian lamin filaments is in agreement with both parallel and antiparallel lateral assembly. A non-uniform coverage of the inner nuclear membrane (INM) consists of dense lamin network regions separated by sparse areas. On average, only half of the total area of the NL appears to be occupied by lamins, which are constrained within a thin layer of 14 nm underneath the INM [18]. Based on these results, the NL appears to form a thin and sparse filamentous sheet that reflects the contribution of the interactions with chromosomes to nuclear mechanics over its own stiffness (discussed below).

Direct transmission of forces to the nucleus Lacking a direct attachment between the cell nucleus and the extracellular environment, the transmission of forces into the nucleus and the activation of nuclear mechanosensory elements is mediated via the Linker of a Nucleoskeleton and Cytoskeleton (LINC) complex [20,21]. The c-termini of the trimeric SUN1 and SUN2 INM proteins and the outer nuclear membrane (ONM) Nesprin proteins extend into the perinuclear lumen and bind via the SUN-KASH domain association to form a physical bridge between the nucleoskeleton and the cytoskeleton. These nuclear attachments are specific to the main cytoskeletal networks of the cell. Nesprin-1 and 2 bind F-actin via the N-terminal Calponin Homology domain, Nesprin-3 binds intermediate filaments (IFs) via plectin, and the shortest isoform Nesprin-4 binds kif5b, a subunit of the microtubule motor protein kinesin-1 [22]. Analogous to cell–matrix and cell–cell adhesions, the LINC complex can thus be regarded as a nuclear adhesion element that enables the rapid propagation of tensile forces from matrix to nucleus and back along the cytoskeleton [23]. The deformation of the nucleus due to applied physical stresses is discussed below. Nucleus deformability dictates mechanosensitive response to fast and slow physical inputs Cells experience a broad range of forces that change in magnitude and in time. The mechanical response of the nucleus is important for elucidating mechanisms of how mechanotransduction signaling is mediated. The nucleus lacks a stable three-dimensional fibrous network that transverses the nucleoplasm and confers shape and mechanical integrity, akin to the cytoskeleton [24–26]. The two main mechanical elements are the NL and the tightly packed interphase chromosomes. Nucleus deformability varies greatly between cell types and it decreases during embryonic stem cell differentiation [27,28]. Yet, the interphase nucleus of solid tissue cells is generally stiffer and more viscous than the cytoplasm [29,30]. As a result, nucleus deformability becomes the main obstacle to cell migration across physically constrained spaces and is modulated by A-type lamins that confer physical protection of the genome [31–34]. Current Opinion in Cell Biology 2017, 46:119–127

Excessive shear stresses exerted during the migration of cancer and immune cells across narrow constrictions can lead to NE rupture, DNA breaks, chromosome mislocalization and recruitment of the molecular repair machinery [35,36,37]. Depletion of lamin levels increased the likelihood of NE rupture, thus increasing genome instability and chromosomal aberrations. Similarly, nuclear tension can stimulate the recruitment of lipid processing enzymes into the stretched nuclear membrane to activate a proinflammatory lipid signaling [38]. Nucleus rheology is complex owing to its heterogeneous structural organization [39] and the broad range of strong and transient interactions with chromatin [40]. Despite this complexity, the global mechanical response to constant micropipette aspiration was well modeled using a simple arrangement of two springs and one dashpot (the SLS model), accounting for a single dissipative relaxation mode [30]. This viscoelastic behavior shifted towards an elastic load-bearing system in swollen nuclei where chromatin attachments are disrupted [41,42]. Three-dimensional chromatin cross-interactions were consistent with a power-low rheology, accounting for a broad range of timescales and relaxation modes that cannot be modeled using simple spring-dashpot circuits [27]. Viscosity is conferred mostly by lamin-A/C, whereas the B-type lamina generates an elastic resistance to stretching [33,43]. Viscosity is consistent with the presence of a 5–10% nucleoplasmic fraction of mature lamin-A/C in interphase cells compared with undetectable nucleoplasmic levels of the farnesylated B-type lamins [44]. The exact multimeric nature of nucleoplasmic lamin-A/C is not clear but has been implicated in generating transient interactions with chromatin with no detectable sequence dependence [45,46]. Lamins also form stable attachments with long genomic regions termed laminaassociated domains (LADs). LADs make up 35% of the genome and are generally associated with a transcriptionally repressive environment [47]. Constitutive LADs (cLADs) are conserved across species in terms of genomic location and size but not sequence. Long A/T-rich segments seem to serve as docking signals to the lamina, [48] indicative of a functional genome organization. Likewise, nucleolus-associated domains (NADs) form nucleoplasmic chromosomal crosslinks [49].

Conversion of mechanical forces into biochemical signals Unlike molecular recognition motifs, which have evolved to specifically target protein domains, chromatin modifications and nucleic acid sequences, mechanical forces act on all interconnected components as they propagate along the cytoskeleton and into the nucleus. Yet, only specific structural elements can convert forces into biochemical signals that are anchored into force-bearing hotspots and are prone to undergo forced unfolding and exposure of functional cryptic sites or shielding of others. In this www.sciencedirect.com

Nuclear mechanotransduction: sensing the force from within Athirasala, Hirsch and Buxboim 121

manner, mechanically induced conformational changes can promote or hinder molecular interactions with signaling molecules by increasing or decreasing their allosteric accessibility. This mechanobiochemical regulation is based on modulating existing molecular interactions, thus rendering molecular specificity while triggering or blocking downstream signaling pathways. Exposure of cryptic molecular recognition sites by forced unfolding appears to be a unifying principle that applies to extracellular filaments, cell–matrix and cell–cell adhesions, and cytoskeletal proteins (Figure 1). Stretching of fibronectin facilitates an extracellular fibril assembly [50– 52]. Stretching of integrins modulate ligand binding [53,54], extension of p130Cas increases the rate of phosphorylation by Src kinase [55], and extension of talin facilitates the recruitment and binding of vinculin [56] necessary for mechanically-induced focal adhesion growth and activation of downstream Rho-ROCK signaling and other pathways [57–59]. Similarly, strain-induced proliferation of epithelial monolayers is mediated by activation and nuclear translocation of E-cadherinsequestered Yap1 and b-catenin [60]. Forced unfolding of the cytoskeletal components spectrin, vimentin and myosin-IIA was shown to regulate their structural organization, polymerization and assembly [61]. Similar to cell-matrix adhesions, LINC complex assemblies form dynamic nuclear attachments that undergo structural remodeling and local stiffening in response to applied force [62]. Local stiffening of isolated nuclei was observed only when pulling forces were mediated

through the LINC complex using magnetic beads coated with anti-nesprin-1 antibodies. Stiffening required laminA/C and was associated with Tyr74 and Tyr95 phosphorylation of the INM protein emerin. Local stiffening is thus attributed to tension-dependent reinforcement of cytoskeletal-nucleoskeletal interactions, which facilitates an efficient transmission of tensile forces inside-out and outside-in [23]. The forces that are applied to the NL unfold the Ig-domain of lamin-A/C, as was demonstrated using isolated nuclei exposed to controlled shear, thus protecting the nucleus from rupture [43]. Moreover, nuclear tensile forces suppress lamin-A/C phosphorylation, including the so-called mitotic site Serine-22 that triggers disassembly [44]. Tension-dependent suppression of lamin-A/C phosphorylation is consistent with a mechanically induced intermolecular steric shielding that hinders kinase accessibility (Figure 1). Consistently, cell relaxation induced by matrix softness, adhesion detachment or pharmaceutical inhibitors or myosin contractility independently show a significant increase in lamin-A/C phosphorylation in interphase cells [44]. As a result, the nucleoplasmic fraction of lamin-A/C is enriched and facilitates easier degradation and faster turnover rates. Force-dependent phosphorylation constitutes a nuclear mechanotransduction mechanism, which provides the cell a means for regulating its NL and activating pathways downstream of nucleoplasmic lamin-A/C [43,44]. In addition to lamin-A/C, tensile forces were associated with conferring protection from proteolytic degradation of extracellular collagen fibrils [63–65], whereas cell relaxation enhances disassembly of the cytoskeletal

Figure 1

Low tension

High tension

Protein forced unfolding

Tension-dependent filament stabilization Chromatin stretching Current Opinion in Cell Biology

The conversion of mechanical forces into biochemical cues can be performed by specific mechanisms. Forced unfolding of linker and adapter proteins exposes cryptic sites that become available for binding and activating interacting factors (top). Tension that is applied to rope-like supercoiled filaments hinders the accessibility of signaling molecules, thus conferring protection from induce disassembly, protein turnover and proteolytic degradation (middle). The propagation of forces across the LINC complex attachments and transmission into the nucleus can directly pull on chromatin segments, thus increasing their binding affinity to RNA-polymerase and the transcriptional machinery (bottom).

www.sciencedirect.com

Current Opinion in Cell Biology 2017, 46:119–127

122 Cell nucleus

intermediate filament vimentin [61] and non-muscle myosin-IIA minifilament [44,66]. Collectively, exerted forces are associated with ECM stabilization and stiffening that were shown to promote cell-generated traction forces [67–69]. Cytoskeletal strain will further increase due to lamin-A/C stabilization and nuclear stiffening, thus forming a stress concentrator [70]. Cytoskeletal assembly and internal prestress were shown to adapt to matrix elasticity [71], in accordance with lamin-A/C expression levels that matched tissue microelasticity [43]. This constitutes a mechanobiochemical feedback, which maintains a contractile cell state, typical of muscle, cartilage or bone and other tissues under mechanical load (Figure 2).

Molecular mediators of cellular mechanotransduction The conversion of mechanical inputs into biochemical signals is performed by mechanisms such as forced unfolding of adaptor and linker proteins, and tensiondependent stabilization of protein filaments (Figure 1). Mechanoregulation of cellular functions requires a means for propagating these mechano-converted signals via the available repertoire of signaling pathways (Figure 3). Several molecular mediators were discovered recently, which share a common principle: their cytoplasmic-tonuclear translocation is stimulated by mechanical inputs. MKL1 is a transcription coactivator of the serum response factor (SRF) transcription factor, which is sequestered by cytoplasmic G-actin. Serum supplementation [72], applied mechanical stress and other physical inputs that induce a contractile cell state [73–75] facilitate nuclear accumulation of MKL1, concomitantly with the

polymerization of G-actin into polymerized F-actin. In the nucleus, export of MKL1 is modulated by nuclear actin [76], where lamin-A/C and emerin, which is an actincapping protein, promote nuclear actin polymerization to retain nucleoplasmic localization and activation of SRFtarget genes [77]. Interestingly, applied strain can also drive the translocation of INM emerin to the ONM, where it promotes the formation of perinuclear actomyosin filaments, the depletion of nuclear G-actin and heterochromatin remodeling [78]. SRF is a master regulator of structural and motor proteins of the actomyosin cytoskeleton and the contractile machinery of the cell [76,79,80]. Indeed, a combined transcriptomic-proteomic profiling in MSCs confirmed the impact of lamin-A/C knockdown on MKL1-SRF target genes [44]. Yap/Taz are mechanically-activated transcriptional co-factors of the Hippo pathway, which is implicated with cell–cell contact inhibition, regulation of organ size and cancer progression [81]. Just like MKL1, Yap/Taz translocates to the nucleus in response to matrix stiffness, cell spreading and applied stretching [82,83]. Its nuclear localization and activation are mechanically regulated via the non-canonical Hippo pathway independent of MST-LAST activation. No direct lamin involvement was reported to date, yet capping and severing factors of the actin cytoskeleton were shown to inhibit Yap/Taz mechanotransduction signaling only in mechanically-relaxed cells [83]. In the nucleus, Yap/Taz activate the TEAD family of transcription factors that target proliferative and apoptosis-inhibitor genes. Other mechanoregulated nuclear mediators were identified recently. SHP2 interacts with and acts downstream of Yap/Taz and enhances Wnt signaling [84]. The

Figure 2

LINC stabilization Small G-proteins signaling

Matrix stablization & stiffness

Matrix adhesion growth

Ctyskeletal assembly & contractility Nuclear lamina stabilization & stiffness SRF signaling

Retinoic acid signaling

Current Opinion in Cell Biology

A mechanobiochemical feedback between matrix, cytoskeleton and nucleus maintains a contractile cell state. The stabilization and growth of cellmatrix and cell-nucleus attachments facilitate an efficient propagation of forces inside out and outside in. These forces, together with the indicated signaling pathways stabilize and promote the assembly of matrix, actomyosin and nuclear lamin filaments and increase cell contractility. Cytoskeletal contractile forces induce matrix and nucleus deformations that in turn generate elastic restoring forces. Solid and dashed arrows represent molecular signaling and mechanical stresses, respectively.

Current Opinion in Cell Biology 2017, 46:119–127

www.sciencedirect.com

Nuclear mechanotransduction: sensing the force from within Athirasala, Hirsch and Buxboim 123

Figure 3

Cytoplasm

Y T

Tension

MKL1

n io s n Te

Nkx2.5 RG

Y T TEAD

Nucleoplasm Lamin-A/C Lamin-B1/B2 Nesprin SUN Nuclear pore Cytoskeleton

Nkx2.5

MKL1

SRF

RG RX

E Y T

Emerin Phosphate Tap/Taz

RG RX RARG-RXR

Degradation

Retinoic acid Current Opinion in Cell Biology

Transcriptional regulators translocate to and from the nucleus in accordance with the contractile state of the cell. Tensile forces that are exerted by the cytoskeleton strengthen the LINC complex and stabilize the nuclear lamina by suppressing lamin-A/C phosphorylation, disassembly and degradation. Blue and orange arrows and transcribed genes correspond to relaxed and contractile cell states, respectively.

retinoic acid receptor gamma (RARG) upregulates the transcription of the lamin-A/C gene whereas lamin-A/C protein contributes to the nuclear localization and activation of RARG, optionally via SUN2 [43]. Lastly, matrix softness rather than stiffness stimulates the nuclear localization of Nkx2.5 where it suppresses genes associated with the contractile cell state such as smooth

muscle actin in MSC [85]. Collectively, cell mechanotransduction is mediated via molecular relays that regulate gene programs and tune the contractile state of the cell (Figure 3). LAP2a is another transcriptional and epigenetic regulator that interacts with nucleoplasmic lamin-A/C, yet with currently unknown mechanobiological roles (see Box 1).

Box 1 Lamin-associated polypeptide-2a (Lap2a) gene regulation downstream of lamin-A/C LAP2 isoforms, whose role in chromatin organization and transcription regulation is established, were not directly implicated to date with mechanotransduction signaling. Nevertheless, LAP2a interact with nucleoplasmic lamin-A/C to regulate E2F/pRb signaling [90,91] and the Hippo pathway transcription factor TEAD4 was identified as one of its targets [92]. LAP2-chromatin interactions are mediated either by the Barrier to Autointegration Factor (BAF) via the LAP2–emerin–MAN1 (LEM) domain or directly through the LEM-like domain (both are N-terminal domains) [93]. The association with BAF facilitates peripheral heterochromatin anchorage to the NE in interphase cells [94]. However, LAP2a differs from the other alternatively spliced isoforms in that it lacks a membrane-binding domain. Instead, LAP2a encodes a unique C-terminal tail that binds exclusively to A-type lamins [95] and promotes nucleoplasmic localization of lamin-A/C [91]. Nucleoplasmic complexes of lamin-A/C and Lap2a bind the retinoblastoma protein (pRb) and stabilize it in the nucleus [90]. In its active or hypo-phosphorylated form, pRb negatively regulates cell cycle progression by binding to the E2F transcription factor and cyclin-D3, causing cell-cycle arrest in the G1-phase [96,97]. Chromatin immunoprecipitation (ChIP) profiles indicate that lamin-A/C and LAP2a associate with overlapping euchromatic regions and loss of LAP2a drives relocalization of lamin-A/C from euchromatin towards heterochromatin regions [98]. These chromatin associations suggest that LAP2a-lamin-A/C interactions play a role in maintaining open euchromatin configurations. Nucleoplasmic lamin-A/C was reported to also retain Polycomb-mediated gene silencing [78,99]. Conceivably, loss of or mutations in either LAP2a or lamin-A/C are associated with increased cell cycling, hyperproliferation [100–102] and premature aging-related disease phenotypes such as Hutchinson Gilford Progeria Syndrome (HGPS) [103–105]. A mechanobiological involvement of LAP2a—lamin-A/C interactions in regulating cell cycle of progenitor and stem cell functions is yet to be evaluated, yet based on these observations it is tempting to speculate that a mechanoresponsive gain or loss of nucleoplasmic lamins and/or structural remodeling of the NL could affect gene expression via LAP2a-related pathways.

www.sciencedirect.com

Current Opinion in Cell Biology 2017, 46:119–127

124 Cell nucleus

response, and dormancy in hepatocellular carcinoma cells. Hepatology 2011, 53:1192-1205.

Force-dependent genome reorganization modulates transcription The genome of interphase nucleus is organized into discrete chromosome territories [86] and substructures termed topologically associated domains [87]. Lineage specific genome organization is associated with transcription regulation.[88] This motivates the hypothesis that mechanical forces, which physically deform the nucleus, can regulate gene expression independent of molecular relays by directly stretching chromosomal sections and/or opening condensed configurations (Figure 1). If so, mechanobiological functions will require mechanisms to target specific genes in a cell-type dependent manner. Tajik et al. exerted periodic shear stress to the apical surface of CHO cells using magnetic beads and measured the extent of chromatin stretching and the rate of transcription of an integrated transgene [89]. Indeed, transcription rate correlated with stress-induced chromatin stretching and both depended on the magnitude of applied shear stress and angle. Stress-induced chromatin stretching and transcription was disrupted when nuclear lamins, emerin, LINC complex proteins, heterochromatin protein-1 (HP1) and BAF were knocked down but not lamin-B receptor (LBR). Moreover, stress-induced chromatin stretching and transcription was amplified in contractile cells and was suppressed in relaxed cells. The effects of applied stress did not depend on the activation of cell-matrix adhesion signaling because comparable induced transcription rates were measured for beads coated with RGD peptide and nonspecifically with PLL that induced similar chromatin stretching. Hence, mechanotransdcution is facilitated here by force propagation along the actomyosin cytoskeleton, LINC attachments, nuclear lamins and heterochromatin linkers to induce chromatin stretching and increase the accessibility of the transcriptional machinery [89].

Acknowledgements This work was supported by the Israel Science Foundation (1246/14) and the Niedersachsen Vorab fund (A115548).

References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as:  of special interest  of outstanding interest 1.

Discher DE, Janmey P, Wang YL: Tissue cells feel and respond to the stiffness of their substrate. Science 2005, 310:1139-1143.

2.

DuFort CC, Paszek MJ, Weaver VM: Balancing forces: architectural control of mechanotransduction. Nat. Rev. Mol. Cell Biol. 2011, 12:308-319.

3.

Pelham RJ, Wang YL: Cell locomotion and focal adhesions are regulated by substrate flexibility. Proc. Natl. Acad. Sci. U. S. A. 1997, 94:13661-13665.

4.

Schrader J, Gordon-Walker TT, Aucott RL, van Deemter M, Quaas A, Walsh S, Benten D, Forbes SJ, Wells RG, Iredale JP: Matrix stiffness modulates proliferation, chemotherapeutic

Current Opinion in Cell Biology 2017, 46:119–127

5.

Engler AJ, Sen S, Sweeney HL, Discher DE: Matrix elasticity directs stem cell lineage specification. Cell 2006, 126:677-689.

6.

Zoldan J, Karagiannis ED, Lee CY, Anderson DG, Langer R, Levenberg S: The influence of scaffold elasticity on germ layer specification of human embryonic stem cells. Biomaterials 2011, 32:9612-9621.

7.

Chowdhury F, Li YZ, Poh YC, Yokohama-Tamaki T, Wang N, Tanaka TS: Soft substrates promote homogeneous selfrenewal of embryonic stem cells via downregulating cellmatrix tractions. PLoS One 2010, 5.

8.

Samarage CR, White MD, Alvarez YD, Fierro-Gonzalez JC, Henon Y, Jesudason EC, Bissiere S, Fouras A, Plachta N: Cortical tension allocates the first inner cells of the mammalian embryo. Dev. Cell 2015, 34:435-447.

9.

Maitre JL, Turlier H, Illukkumbura R, Eismann B, Niwayama R, Nedelec F, Hiiragi T: Asymmetric division of contractile domains couples cell positioning and fate specification. Nature 2016, 536:344-348.

10. Aebi U, Cohn J, Buhle L, Gerace L: The nuclear lamina is a meshwork of intermediate-type filaments. Nature 1986, 323:560-564. 11. Stuurman N, Heins S, Aebi U: Nuclear lamins: their structure, assembly, and interactions. J. Struct. Biol. 1998, 122:42-66. 12. Goldman RD, Gruenbaum Y, Moir RD, Shumaker DK, Spann TP: Nuclear lamins: building blocks of nuclear architecture. Genes. Dev. 2002, 16:533-547. 13. Gruenbaum Y, Margalit A, Goldman RD, Shumaker DK, Wilson KL: The nuclear lamina comes of age. Nat. Rev. Mol. Cell Biol. 2005, 6:21-31. 14. Shimi T, Pfleghaar K, Kojima SI, Pack CG, Solovei I, Goldman AE, Adam SA, Shumaker DK, Kinjo M, Cremer T et al.: The A- and Btype nuclear lamin networks: microdomains involved in chromatin organization and transcription. Genes Dev. 2008, 22:3409-3421. 15. Shimi T, Kittisopikul M, Tran J, Goldman AE, Adam SA, Zheng YX, Jaqaman K, Goldman RD: Structural organization of nuclear lamins A, C, B1, and B2 revealed by superresolution microscopy. Mol. Biol. Cell 2015, 26:4075-4086. 16. Harapin J, Bormel M, Sapra KT, Brunner D, Kaech A, Medalia O: Structural analysis of multicellular organisms with cryoelectron tomography. Nat. Methods 2015, 12:634-636. 17. Mahamid J, Pfeffer S, Schaffer M, Villa E, Danev R, Cuellar LK,  Forster F, Hyman AA, Plitzko JM, Baumeister W: Visualizing the molecular sociology at the HeLa cell nuclear periphery. Science 2016, 351:969-972. The implementation of the advances in Cryo-TM technologies enabled 3D molecular visualization in Hela cells. With regard to this review, Mahamid et al. provide high-resolution images of the nucleoplasmic and the cytoplasmic sides of the NE. This work reveals 4 nm diameter filaments at the nuclear lamina, that are consistent with previous measurements of nuclear lamins in nematodes, and detailed description of how these filaments associate with nuclear pores and relative to the cytoskeletal elements. 18. Turgay Y, Eibauer M, Goldman AE, Shimi T, Khayat M, Ben Harush K, Dubrovsky-Gaupp A, Sapra KT, Goldman RD, Medalia O: The molecular architecture of lamins in somatic cells. Nature 2017, 543:261-264. A Cryo-TM analysis reveals a detailed structural description of nuclear lamins and lamina organization. Lamin filaments are 3.5 nm thick, they extend over 400 nm in length and exhibit high flexibilily with persistence length as short as 50 nm. The network of lamin filaments is confined into a thin layer (14 nm) that covers only half of the inner membrane surface and consist of dense and sparse regions. This is the most detalied study of nuclear lamina organization and lamin assembly. 19. Ben-Harush K, Wiesel N, Frenkiel-Krispin D, Moeller D, Soreq E, Aebi U, Herrmann H, Gruenbaum Y, Medalia O: The supramolecular organization of the C. elegans nuclear lamin filament. J. Mol. Biol. 2009, 386:1392-1402. www.sciencedirect.com

Nuclear mechanotransduction: sensing the force from within Athirasala, Hirsch and Buxboim 125

20. Padmakumar VC, Libotte T, Lu WS, Zaim H, Abraham’ S, Noegel AA, Gotzmann J, Foisner R, Karakesisoglou L: The inner nuclear membrane protein Sun1 mediates the anchorage of Nesprin-2 to the nuclear envelope. J. Cell Sci. 2005, 118: 3419-3430. 21. Crisp M, Liu Q, Roux K, Rattner JB, Shanahan C, Burke B, Stahl PD, Hodzic D: Coupling of the nucleus and cytoplasm: role of the LINC complex. J. Cell Biol. 2006, 172:41-53. 22. Rajgor D, Shanahan CM: Nesprins: from the nuclear envelope and beyond. Expert Rev. Mol. Med. 2013, 15. 23. Buxboim A, Ivanovska IL, Discher DE: Matrix elasticity, cytoskeletal forces and physics of the nucleus: how deeply do cells ‘feel’ outside and in? J. Cell Sci. 2010, 123:297-308.

Induced genomic instability and chromosomeaberrations may contribute to cancer cell progression and impact immune response and pathology. 36. Raab M, Gentili M, de Belly H, Thiam HR, Vargas P, Jimenez AJ,  Lautenschlaeger F, Voituriez R, Lennon-Dumenil AM, Manel N et al.: ESCRT III repairs nuclear envelope ruptures during cell migration to limit DNA damage and cell death. Science 2016, 352:359-362. See above (Denais et al.). 37. Irianto J, Xia YT, Pfeifer CR, Athirasala A, Ji JZ, Alvey C, Tewari M,  Bennett RR, Harding SM, Liu AJ et al.: DNA damage follows repair factor depletion and portends genome variation in cancer cells after pore migration. Curr. Biol. 2017, 27:210-223. See above (Denais et al.).

25. Wilson RH, Hesketh EL, Coverley D: The nuclear matrix: fractionation techniques and analysis. Cold Spring Harb. Protoc. 2016, 2016 pdb top074518.

38. Enyedi B, Jelcic M, Niethammer P: The cell nucleus serves as a  mechanotransducer of tissue damage-induced inflammation. Cell 2016, 165:1160-1170. Osmotic cell swelling due to tissue damage increases nuclear tension and activates proinflammatory lipid signaling that attracts neutrophils to these sites. This work presents a novel mechanism of nuclear mechanosensing that depends on perinuclear actin and lamin-A/C.

26. Hancock R: A new look at the nuclear matrix. Chromosoma 2000, 109:219-225.

39. Lammerding J: Mechanics of the nucleus. Compr. Physiol. 2011, 1:783-807.

27. Pajerowski JD, Dahl KN, Zhong FL, Sammak PJ, Discher DE: Physical plasticity of the nucleus in stem cell differentiation. Proc. Natl. Acad. Sci. U. S. A. 2007, 104:15619-15624.

40. Schreiner SM, Koo PK, Zhao Y, Mochrie SGJ, King MC: The tethering of chromatin to the nuclear envelope supports nuclear mechanics. Nat. Commun. 2015, 6.

28. Olins AL, Zwerger M, Herrmann H, Zentgraf H, Simon AJ, Monestier M, Olins DE: The human granulocyte nucleus: unusual nuclear envelope and heterochromatin composition. Eur. J. Cell Biol. 2008, 87:279-290.

41. Dahl KN, Engler AJ, Pajerowski JD, Discher DE: Power-law rheology of isolated nuclei with deformation mapping of nuclear substructures. Biophys. J. 2005, 89:2855-2864.

24. Pederson T: Half a century of the nuclear matrix. Mol. Biol. Cell 2000, 11:799-805.

29. Caille N, Thoumine O, Tardy Y, Meister JJ: Contribution of the nucleus to the mechanical properties of endothelial cells. J. Biomech. 2002, 35:177-187. 30. Guilak F, Tedrow JR, Burgkart R: Viscoelastic properties of the cell nucleus. Biochem. Biophys. Res. Commun. 2000, 269: 781-786. 31. McGregor AL, Hsia CR, Lammerding J: Squish and squeeze—the nucleus as a physical barrier during migration in confined environments. Curr. Opin. Cell Biol. 2016, 40:32-40. 32. Davidson PM, Denais C, Bakshi MC, Lammerding J: Nuclear deformability constitutes a rate-limiting step during cell migration in 3-D environments. Cell. Mol. Bioeng. 2014, 7:293306. 33. Harada T, Swift J, Irianto J, Shin JW, Spinler KR, Athirasala A, Diegmiller R, Dingal PCDP, Ivanovska IL, Discher DE: Nuclear lamin stiffness is a barrier to 3D migration, but softness can limit survival. J. Cell Biol. 2014, 204:669-682. 34. Rowat AC, Jaalouk DE, Zwerger M, Ung WL, Eydelnant IA, Olins DE, Olins AL, Herrmann H, Weitz DA, Lammerding J: Nuclear envelope composition determines the ability of neutrophiltype cells to passage through micron-scale constrictions. J. Biol. Chem. 2013, 288:8610-8618. 35. Denais CM, Gilbert RM, Isermann P, McGregor AL, te Lindert M, Weigelin B, Davidson PM, Friedl P, Wolf K, Lammerding J: Nuclear  envelope rupture and repair during cancer cell migration. Science 2016, 352:353-358. The cell nucleus is the dominant physical mass that limits the migration of cells across constricted spaces that are smaller than the nucleus cross section area. Pulling the trailing nucleus across the narrow rigid pores by myosin contractile forces exerts significant shear forces and increases nuclear pressure that can induce herniation (formation of blebs) and rupture of the nuclear membrane at sites of high curvature and local weakness of the underlying lamina, and permit exchange of cytoplasmic and nucleoplasmic material. This mechanical process was further implicated with DNA breaks, chromosome aberrations and genomic instability, although DNA damage may also be induced by cytoplasmic factors (Raab et al., below), and cytoplasmic displacement of DNA repair proteins. The endosomal sorting complexes required for transport III (ESCRT III) machinery was shown to be involved in resealing the nuclear membrane not only during late anaphase but also in interphase cells. Several cancer cell types (Denais et al. and Irianto et al., below) and immune cells (Raab et al.) express low levels of lamins, which promote the NE rupture and DNA damage in these cells and increase the incidence of cell death. www.sciencedirect.com

42. Dahl KN, Kahn SM, Wilson KL, Discher DE: The nuclear envelope lamina network has elasticity and a compressibility limit suggestive of a molecular shock absorber. J. Cell Sci. 2004, 117:4779-4786. 43. Swift J, Ivanovska IL, Buxboim A, Harada T, Dingal PCDP, Pinter J, Pajerowski JD, Spinler KR, Shin JW, Tewari M et al.: Nuclear lamin-A scales with tissue stiffness and enhances matrixdirected differentiation. Science 2013, 341:1240104 http://dx. doi.org/10.1126/science. 44. Buxboim A, Swift J, Irianto J, Spinler KR, Dingal PCDP, Athirasala A, Kao YRC, Cho S, Harada T, Shin JW et al.: Matrix elasticity regulates lamin-A,C phosphorylation and turnover with feedback to actomyosin. Curr. Biol. 2014, 24:1909-1917. 45. Bronshtein I, Kepten E, Kanter I, Berezin S, Lindner M,  Redwood AB, Mai S, Gonzalo S, Foisner R, Shav-Tal Y et al.: Loss of lamin A function increases chromatin dynamics in the nuclear interior. Nat. Commun. 2015, 6. The dynamics of interphase chromatin segments in live adhering cells exhibit a subdiffusion profile that is indicative of weak and transient binding interactions. However, chromatin dynamics becomes faster and the mean square displacement time dependence corresponds to a normal diffusion in LMNA-null cells. Hence, either lamin-A/C is responsible for the expression of interacting proteins (unlikely) or lamin-A/C itself (directly or indirectly) serves as a glue that weakly crosslinks neighboring DNA and/or chromatin segments. These findings provide insights into the potential role of nucleoplasmic lamin-A/C in maintaining nuclear organization, controlling the mechanical response of the nucleus to applied forces and regulating gene expression with implications to lamin-A/C depended disease mechanisms. 46. Bronshtein I, Kanter I, Kepten E, Lindner M, Berezin S, Shav-Tal Y, Garini Y: Exploring chromatin organization mechanisms through its dynamic properties. Nucleus 2016, 7:27-33. 47. Guelen L, Pagie L, Brasset E, Meuleman W, Faza MB, Talhout W, Eussen BH, de Klein A, Wessels L, de Laat W et al.: Domain organization of human chromosomes revealed by mapping of nuclear lamina interactions. Nature 2008, 453:948-951. 48. Meuleman W, Peric-Hupkes D, Kind J, Beaudry JB, Pagie L, Kellis M, Reinders M, Wessels L, van Steensel B: Constitutive nuclear lamina-genome interactions are highly conserved and associated with A/T-rich sequence. Genome Res. 2013, 23:270280. 49. Nemeth A, Conesa A, Santoyo-Lopez J, Medina I, Montaner D, Peterfia B, Solovei I, Cremer T, Dopazo J, Langst G: Initial genomics of the human nucleolus. PLoS Genet. 2010, 6. Current Opinion in Cell Biology 2017, 46:119–127

126 Cell nucleus

50. Krammer A, Lu H, Isralewitz B, Schulten K, Vogel V: Forced unfolding of the fibronectin type III module reveals a tensile molecular recognition switch. Proc. Natl. Acad. Sci. U. S. A. 1999, 96:1351-1356. 51. Smith ML, Gourdon D, Little WC, Kubow KE, Eguiluz RA, LunaMorris S, Vogel V: Force-induced unfolding of fibronectin in the extracellular matrix of living cells. PLoS Biol. 2007, 5: 2243-2254. 52. Baneyx G, Baugh L, Vogel V: Fibronectin extension and unfolding within cell matrix fibrils controlled by cytoskeletal tension. Proc. Natl. Acad. Sci. U. S. A. 2002, 99:5139-5143. 53. Chen W, Lou JZ, Evans EA, Zhu C: Observing force-regulated conformational changes and ligand dissociation from a single integrin on cells. J. Cell Biol. 2012, 199:497-512. 54. Friedland JC, Lee MH, Boettiger D: Mechanically activated integrin switch controls alpha(5)beta(1) function. Science 2009, 323:642-644. 55. Sawada Y, Tamada M, Dubin-Thaler BJ, Cherniavskaya O, Sakai R, Tanaka S, Sheetz MP: Force sensing by mechanical extension of the Src family kinase substrate p130Cas. Cell 2006, 127:1015-1026. 56. del Rio A, Perez-Jimenez R, Liu RC, Roca-Cusachs P, Fernandez JM, Sheetz MP: Stretching single talin rod molecules activates vinculin binding. Science 2009, 323:638-641. 57. Janostiak R, Pataki AC, Brabek J, Rosel D: Mechanosensors in integrin signaling: the emerging role of p130Cas. Eur. J. Cell Biol. 2014, 93:445-454. 58. Geiger B, Spatz JP, Bershadsky AD: Environmental sensing through focal adhesions. Nat. Rev. Mol. Cell Biol. 2009, 10: 21-33. 59. Huveneers S, Danen EHJ: Adhesion signaling—crosstalk between integrins, Src and Rho. J. Cell Sci. 2009, 122: 1059-1069. 60. Benham-Pyle BW, Pruitt BL, Nelson WJ: Mechanical strain  induces E-cadherin-dependent Yap1 and beta-catenin activation to drive cell cycle entry. Science 2015, 348:10241027. This work highlight the mechanobiological function of cell–cell E-cadherin junctions. Cytoplasmic Yap1 and b-catenin that are sequestered to cell–cell junctions of epithelial monolayers are released in response to induced strain that is applied by matrix stretching to the E-cadherin extracellular domain binding. This mechanical signal is mediated by the translocation of Yap1 and b-catenin to the nucleus and downstream transcriptional activation, which are responsible for strain-dependent cell–cycle reentry and G1-to-S phase progression. 61. Johnson CP, Tang HY, Carag C, Speicher DW, Discher DE: Forced unfolding of proteins within cells. Science 2007, 317:663-666. 62. Guilluy C, Osborne LD, Van Landeghem L, Sharek L, Superfine R, Garcia-Mata R, Burridge K: Isolated nuclei adapt to force and reveal a mechanotransduction pathway in the nucleus. Nat. Cell Biol. 2014, 16:376-381. 63. Flynn BP, Bhole AP, Saeidi N, Liles M, DiMarzio CA, Ruberti JW: Mechanical strain stabilizes reconstituted collagen fibrils against enzymatic degradation by mammalian collagenase matrix metalloproteinase 8 (MMP-8). PLoS One 2010, 5. 64. Bhole AP, Flynn BP, Liles M, Saeidi N, Dimarzio CA, Ruberti JW: Mechanical strain enhances survivability of collagen micronetworks in the presence of collagenase: implications for load-bearing matrix growth and stability. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 2009, 367:3339-3362. 65. Ruberti JW, Hallab NJ: Strain-controlled enzymatic cleavage of collagen in loaded matrix. Biochem. Biophys. Res. Commun. 2005, 336:483-489. 66. Raab M, Swift J, Dingal PCDP, Shah P, Shin JW, Discher DE: Crawling from soft to stiff matrix polarizes the cytoskeleton and phosphoregulates myosin-II heavy chain. J. Cell Biol. 2012, 199:669-683.

Current Opinion in Cell Biology 2017, 46:119–127

67. Lo CM, Wang HB, Dembo M, Wang YL: Cell movement is guided by the rigidity of the substrate. Biophys. J. 2000, 79:144-152. 68. Paszek MJ, Zahir N, Johnson KR, Lakins JN, Rozenberg GI, Gefen A, Reinhart-King CA, Margulies SS, Dembo M, Boettiger D et al.: Tensional homeostasis and the malignant phenotype. Cancer Cell 2005, 8:241-254. 69. Saez A, Buguin A, Silberzan P, Ladoux B: Is the mechanical activity of epithelial cells controlled by deformations or forces? Biophys. J. 2005, 89:L52-L54. 70. Heo SJ, Driscoll TP, Thorpe SD, Nerurkar NL, Baker BM, Yang MT,  Chen CS, Lee DA, Mauck RL: Differentiation alters stem cell nuclear architecture, mechanics, and mechanosensitivity. Elife 2016, 5. Stem cell differentiation decreases nucleus deformability due to increasing chromatin condensation and lamin-A/C expression and assembly. As a result, the nucleus transitions from serving as a strain sink towards a strain concentrator—namely the differentiated nucleus deforms less under applied strain at the expense of increased cytoplasmic strain. This makes the differentiated cell more mechanosensitive to applied stretch, which can be mediated by the release of cytoplasmic Calcium stores. This work highlights a mechanical feedback by which physical nuclear changes alter cell mechanosensitivity. 71. Solon J, Levental I, Sengupta K, Georges PC, Janmey PA: Fibroblast adaptation and stiffness matching to soft elastic substrates. Biophys. J. 2007, 93:4453-4461. 72. Miralles F, Posern G, Zaromytidou AI, Treisman R: Actin dynamics control SRF activity by regulation of its coactivator MAL. Cell 2003, 113:329-342. 73. Somogyi K, Rorth P: Evidence for tension-based regulation of Drosophila MAL and SRF during invasive cell migration. Dev. Cell 2004, 7:85-93. 74. Iyer KV, Pulford S, Mogilner A, Shivashankar GV: Mechanical activation of cells induces chromatin remodeling preceding MKL nuclear transport. Biophys. J. 2012, 103:1416-1428. 75. Connelly JT, Gautrot JE, Trappmann B, Tan DWM, Donati G, Huck WTS, Watt FM: Actin and serum response factor transduce physical cues from the microenvironment to regulate epidermal stem cell fate decisions. Nat. Cell Biol. 2010, 12:711-718. 76. Vartiainen MK, Guettler S, Larijani B, Treisman R: Nuclear actin regulates dynamic subcellular localization and activity of the SRF cofactor MAL. Science 2007, 316:1749-1752. 77. Ho CY, Jaalouk DE, Vartiainen MK, Lammerding J: Lamin A/C and emerin regulate MKL1-SRF activity by modulating actin dynamics. Nature 2013, 497:507-511. 78. Le HQ, Ghatak S, Yeung CYC, Tellkamp F, Gunschmann C,  Dieterich C, Yeroslaviz A, Habermand B, Pombo A, Niessen CM et al.: Mechanical regulation of transcription controls Polycomb-mediated gene silencing during lineage commitment. Nat. Cell Biol. 2016, 18:864-875. External forces that propagate and pull on the nuclear lamina favor the translocation of emerin from the INM to the ONM. The depletion of INM emerin leads to loss of constitutive heterochromatin marks. In turn, the enrichment of ONM emerin, which is a potent actin capping protein, supports the polymerization of perinuclear actomyosin stress fibers and the depletion of nuclear monomeric actin. Together, applied nuclear forces lead to heterochromatin reorganization, attenuation of RNA-Pol II transcriptional elongation and gene silencing. This mechanism highlights the mechanosensing properties of emerin and reveals means for regulating gene expression and directing differentiation pathways in stem cells under mechanical load. 79. Sun Q, Chen G, Streb JW, Long XC, Yang YM, Stoeckert CJ, Miano JM: Defining the mammalian CArGome. Genome Res. 2006, 16:197-207. 80. Olson EN, Nordheim A: Linking actin dynamics and gene transcription to drive cellular motile functions. Nat. Rev. Mol. Cell Biol. 2010, 11:353-365. 81. Low BC, Pan CQ, Shivashankar GV, Bershadsky A, Sudol M, Sheetz M: YAP/TAZ as mechanosensors and mechanotransducers in regulating organ size and tumor growth. FEBS Lett. 2014, 588:2663-2670. www.sciencedirect.com

Nuclear mechanotransduction: sensing the force from within Athirasala, Hirsch and Buxboim 127

82. Dupont S, Morsut L, Aragona M, Enzo E, Giulitti S, Cordenonsi M, Zanconato F, Le Digabel J, Forcato M, Bicciato S et al.: Role of YAP/TAZ in mechanotransduction. Nature 2011, 474:179-183. 83. Aragona M, Panciera T, Manfrin A, Giulitti S, Michielin F, Elvassore N, Dupont S, Piccolo S: A mechanical checkpoint controls multicellular growth through YAP/TAZ regulation by actin-processing factors. Cell 2013, 154:1047-1059. 84. Tsutsumi R, Masoudi M, Takahashi A, Fujii Y, Hayashi T, Kikuchi I, Satou Y, Taira M, Hatakeyama M: YAP and TAZ, hippo signaling targets, act as a rheostat for nuclear SHP2 function. Dev. Cell 2013, 26:658-665. 85. Dingal PCDP, Bradshaw AM, Cho S, Raab M, Buxboim A, Swift J,  Discher DE: Fractal heterogeneity in minimal matrix models of scars modulates stiff-niche stem-cell responses via nuclear exit of a mechanorepressor. Nat. Mater. 2015, 14:951-960. This report describes the development a simple matrix model for mesenchymal stem cells in culture that recapitulates scar-related phenotypes. The expression level of smooth muscle actin (SMA), which marks intracellular tension and is associated with scar, anti-correlated with the nuclear localization of the cardiac transcription factor Nkx2.5. Unlike other molecular mediators of mechanical signals such as MKL1, Yap/Taz or RARG, the nuclear translocation of Nkx2.5 is associated with mechanically relaxed cell states where it suppresses the expression of SMA. 86. Cremer T, Cremer C: Chromosome territories, nuclear architecture and gene regulation in mammalian cells. Nat. Rev. Genet. 2001, 2:292-301. 87. Dixon JR, Selvaraj S, Yue F, Kim A, Li Y, Shen Y, Hu M, Liu JS, Ren B: Topological domains in mammalian genomes identified by analysis of chromatin interactions. Nature 2012, 485:376380. 88. Neems DS, Garza-Gongora AG, Smith ED, Kosak ST: Topologically associated domains enriched for lineagespecific genes reveal expression-dependent nuclear topologies during myogenesis. Proc. Natl. Acad. Sci. U. S. A. 2016, 113:E1691-E1700. 89. Tajik A, Zhang YJ, Wei FX, Sun J, Jia Q, Zhou WW, Singh R,  Khanna N, Belmont AS, Wang N: Transcription upregulation via force-induced direct stretching of chromatin. Nat. Mater. 2016, 15:1287-1296. Nucleus mechanotransduction can be facilitated without any molecular mediators. In this work, mechanical forces that were applied on the cell membrane via magnetic beads, were transmitted across integrin adhesions, propagated along the cytoskeleton, LINC complex attachments and lamina and pulled on transgene segments to activate their transcription. Transcription activation depended on actomyosin contractility and was correlated with the extent of chromatin stretching. 90. Markiewicz E, Dechat T, Foisner R, Quinlan RA, Hutchison CJ: Lamin A/C binding protein LAP2 alpha is required for nuclear anchorage of retinoblastoma protein. Mol. Biol. Cell 2002, 13:4401-4413. 91. Naetar N, Korbei B, Kozlov S, Kerenyi MA, Dorner D, Kral R, Gotic I, Fuchs P, Cohen TV, Bittner R et al.: Loss of nucleoplasmic LAP2 alpha-lamin A complexes causes erythroid and epidermal progenitor hyperproliferation. Nat. Cell Biol. 2008, 10:13411348.

www.sciencedirect.com

92. Muller H, Bracken AP, Vernell R, Moroni MC, Christians F, Grassilli E, Prosperini E, Vigo E, Oliner JD, Helin K: E2Fs regulate the expression of genes involved in differentiation, development, proliferation, and apoptosis. Genes Dev. 2001, 15:267-285. 93. Cai M, Huang Y, Ghirlando R, Wilson KL, Craigie R, Clore GM: Solution structure of the constant region of nuclear envelope protein LAP2 reveals two LEM-domain structures: one binds BAF and the other binds DNA. EMBO J. 2001, 20:4399-4407. 94. Wilson KL, Foisner R: Lamin-binding proteins. Cold Spring Harb. Perspect. Biol. 2010, 2:a000554. 95. Dechat T, Korbei B, Vaughan OA, Vlcek S, Hutchison CJ, Foisner R: Lamina-associated polypeptide 2alpha binds intranuclear A-type lamins. J. Cell Sci. 2000, 113(Pt. 19):34733484. 96. Hatakeyama M, Weinberg RA: The role of RB in cell cycle control. Prog. Cell Cycle Res. 1995, 1:9-19. 97. Weinberg RA: The retinoblastoma protein and cell cycle control. Cell 1995, 81:323-330. 98. Gesson K, Rescheneder P, Skoruppa MP, von Haeseler A, Dechat T, Foisner R: A-type lamins bind both hetero- and euchromatin, the latter being regulated by lamina-associated polypeptide 2 alpha. Genome Res. 2016, 26:462-473. 99. Marullo F, Cesarini E, Antonelli L, Gregoretti F, Oliva G, Lanzuolo C: Nucleoplasmic lamin A/C and polycomb group of proteins: an evolutionarily conserved interplay. Nucleus 2016, 7:103-111. 100. Naetar N, Korbei B, Kozlov S, Kerenyi MA, Dorner D, Kral R, Gotic I, Fuchs P, Cohen TV, Bittner R et al.: Loss of nucleoplasmic LAP2alpha-lamin A complexes causes erythroid and epidermal progenitor hyperproliferation. Nat. Cell Biol. 2008, 10:1341-1348. 101. Gotic I, Leschnik M, Kolm U, Markovic M, Haubner BJ, Biadasiewicz K, Metzler B, Stewart CL, Foisner R: Laminaassociated polypeptide 2alpha loss impairs heart function and stress response in mice. Circ. Res. 2010, 106:346-353. 102. Gotic I, Schmidt WM, Biadasiewicz K, Leschnik M, Spilka R, Braun J, Stewart CL, Foisner R: Loss of LAP2 alpha delays satellite cell differentiation and affects postnatal fiber-type determination. Stem Cells 2010, 28:480-488. 103. Mounkes LC, Kozlov S, Hernandez L, Sullivan T, Stewart CL: A progeroid syndrome in mice is caused by defects in A-type lamins. Nature 2003, 423:298-301. 104. Bridger JM, Kill IR: Aging of Hutchinson-Gilford progeria syndrome fibroblasts is characterised by hyperproliferation and increased apoptosis. Exp. Gerontol. 2004, 39:717-724. 105. Dechat T, Shimi T, Adam SA, Rusinol AE, Andres DA, Spielmann HP, Sinensky MS, Goldman RD: Alterations in mitosis and cell cycle progression caused by a mutant lamin A known to accelerate human aging. Proc. Natl. Acad. Sci. U. S. A. 2007, 104:4955-4960.

Current Opinion in Cell Biology 2017, 46:119–127