Portal de Periódicos da CAPES

Forcing Entry into the Nucleus

2017; Elsevier BV; Volume: 43; Issue: 5 Linguagem: Inglês

10.1016/j.devcel.2017.11.015

1878-1551

Alexis J. Lomakin, G. Nader, Matthieu Piel,

RNA Research and Splicing

Nuclear pore complexes tightly regulate nucleo-cytoplasmic transport, controlling the nuclear concentration of several transcription factors. In a recent issue of Cell, Elosegui-Artola et al., 2017Elosegui-Artola A. Andreu I. Beedle A.E.M. Lezamiz A. Uroz M. Kosmalska A.J. Oria R. Kechagia J.Z. Rico-Lastres P. Le Roux A.L. et al.Cell. 2017; 171: 1397-1410Abstract Full Text Full Text PDF PubMed Scopus (601) Google Scholar show that nuclear deformation modulates the nuclear entry rates of YAP/TAZ via nuclear pore stretching, clarifying how forces affect gene transcription. Nuclear pore complexes tightly regulate nucleo-cytoplasmic transport, controlling the nuclear concentration of several transcription factors. In a recent issue of Cell, Elosegui-Artola et al., 2017Elosegui-Artola A. Andreu I. Beedle A.E.M. Lezamiz A. Uroz M. Kosmalska A.J. Oria R. Kechagia J.Z. Rico-Lastres P. Le Roux A.L. et al.Cell. 2017; 171: 1397-1410Abstract Full Text Full Text PDF PubMed Scopus (601) Google Scholar show that nuclear deformation modulates the nuclear entry rates of YAP/TAZ via nuclear pore stretching, clarifying how forces affect gene transcription. It has been known for a while that mechanical forces can affect cell fate. Pioneering work from the Ingber lab showed that cell shape and cell mechanics regulate cell survival and proliferation (Chen et al., 1997Chen C.S. Mrksich M. Huang S. Whitesides G.M. Ingber D.E. Science. 1997; 276: 1425-1428Crossref PubMed Scopus (4181) Google Scholar) and that external forces can be directly transmitted to the nucleus (Maniotis et al., 1997Maniotis A.J. Chen C.S. Ingber D.E. Proc. Natl. Acad. Sci. USA. 1997; 94: 849-854Crossref PubMed Scopus (1330) Google Scholar). Later studies extended the reach of mechanical forces to the control of cell differentiation and associated gene expression patterns (reviewed in Bellas and Chen, 2014Bellas E. Chen C.S. Curr. Opin. Cell Biol. 2014; 31: 92-97Crossref PubMed Scopus (67) Google Scholar), showing their importance for tissue development and cancer progression. Twenty years of research, on what is now commonly referred to as mechanotransduction, placed the question of the relationship between mechanical forces and gene expression in the center of attention. More recently, Dupont et al. found that most of the assays used to define mechanotransduction —by modulating cell shape or cell substrate rigidity—were also affecting the localization and thus activity of the transcription factor YAP/TAZ, a major regulator of cell growth and proliferation (Dupont et al., 2011Dupont S. Morsut L. Aragona M. Enzo E. Giulitti S. Cordenonsi M. Zanconato F. Le Digabel J. Forcato M. Bicciato S. et al.Nature. 2011; 474: 179-183Crossref PubMed Scopus (3277) Google Scholar). The regulation of YAP/TAZ localization is known to require the actin cytoskeleton and Myosin II contractility, as well as the LINC complex, which physically couples the nuclear envelope to the cytoskeleton (reviewed in Wang et al., 2009Wang N. Tytell J.D. Ingber D.E. Nat. Rev. Mol. Cell Biol. 2009; 10: 75-82Crossref PubMed Scopus (1264) Google Scholar). Thus, Yap is a major downstream effector of mechanotransduction. However, although these studies clearly identified actin-mediated forces acting on the nucleus as important players in mechanotransduction, how these forces modulate YAP/TAZ localization remained elusive. In a recent issue of Cell, Elosegui-Artola et al., 2017Elosegui-Artola A. Andreu I. Beedle A.E.M. Lezamiz A. Uroz M. Kosmalska A.J. Oria R. Kechagia J.Z. Rico-Lastres P. Le Roux A.L. et al.Cell. 2017; 171: 1397-1410Abstract Full Text Full Text PDF PubMed Scopus (601) Google Scholar suggest a provocative answer to this question. By plating cells on substrates of various rigidities or by deforming the nucleus using an AFM (atomic force microscopy) cantilever, these authors demonstrate that nuclear deformation is sufficient to increase nuclear import of the transcription factor YAP/TAZ even in the absence of an actin network or the LINC complex. The study demonstrates that nuclear flattening, either directly imposed by AFM or indirectly induced by transmission of forces via the cytoskeleton, is a central element of mechanotransduction and that it can act independently of biochemical cues. Understanding mechanotransduction has been challenging due to the multiple molecular layers involved in transducing the forces. To gain insight into the process, Elosegui-Artola et al., 2017Elosegui-Artola A. Andreu I. Beedle A.E.M. Lezamiz A. Uroz M. Kosmalska A.J. Oria R. Kechagia J.Z. Rico-Lastres P. Le Roux A.L. et al.Cell. 2017; 171: 1397-1410Abstract Full Text Full Text PDF PubMed Scopus (601) Google Scholar first dissected the force transmission network by plating the cells on substrates of different rigidity. They were able to show that YAP nuclear translocation occurred over a certain rigidity threshold and that translocation depended on talin (focal adhesion complex protein), actin cytoskeleton integrity, and the LINC complex. The same players were responsible for nuclear deformation and the consequent YAP nuclear translocation upon externally imposed cell stretching. Using mutants of YAP, they also demonstrated that, although YAP phosphorylation and the Hippo signaling pathway could modulate YAP localization, these molecular factors did not affect the rigidity threshold in which YAP nuclear translocation occurred. These results confirmed that the pathway that transmits external forces from the substrate to the nucleus is directly responsible for YAP mechanosensing, but the identity of the actual force sensor was still unclear. Elosegui-Artola et al., 2017Elosegui-Artola A. Andreu I. Beedle A.E.M. Lezamiz A. Uroz M. Kosmalska A.J. Oria R. Kechagia J.Z. Rico-Lastres P. Le Roux A.L. et al.Cell. 2017; 171: 1397-1410Abstract Full Text Full Text PDF PubMed Scopus (601) Google Scholar then carried out a set of experiments to address this question. First they applied an AFM cantilever to directly deform the nuclei of cells plated on a soft substrate. Interestingly, this treatment was sufficient to induce YAP translocation to the nucleus. They then asked whether adhesion, actin, and the LINC complex were required for this effect. When they plated cells on a hard substrate and then suppressed actin filament assembly and talin function, YAP remained cytoplasmic. However, here again, applying a direct force on the nucleus was enough to induce YAP translocation, demonstrating that the nucleus could act as a force sensor independent of adhesion and actin. This elegant biophysical approach allowed the authors to differentiate between the force transmission part of the pathway—involving adhesion, actin, and the LINC complex—and the force sensor, the nucleus. The nuclear structure directly associated with regulating nuclear entry and exit of molecules is the nuclear pore complex. Elosegui-Artola et al., 2017Elosegui-Artola A. Andreu I. Beedle A.E.M. Lezamiz A. Uroz M. Kosmalska A.J. Oria R. Kechagia J.Z. Rico-Lastres P. Le Roux A.L. et al.Cell. 2017; 171: 1397-1410Abstract Full Text Full Text PDF PubMed Scopus (601) Google Scholar beautifully demonstrate that nuclear pores are indeed involved in the regulation of YAP localization upon nuclear deformation. Using fluorescence recovery after photobleaching (FRAP) of GFP-YAP, as well as chemical inhibitors and genetic manipulations affecting nuclear import and export, the authors show that nuclear deformation specifically induces an increase in the YAP import rate. This effect, based on transmission electron microscopy, is likely due to the more open conformation that nuclear pores acquire upon nuclear flattening. Such changes could make nuclear pores sterically more permissive to proteins undergoing nuclear import, suggesting that protein factors other than YAP could also be affected by this mechanism. Indeed, in an impressive last set of experiments, Elosegui-Artola et al., 2017Elosegui-Artola A. Andreu I. Beedle A.E.M. Lezamiz A. Uroz M. Kosmalska A.J. Oria R. Kechagia J.Z. Rico-Lastres P. Le Roux A.L. et al.Cell. 2017; 171: 1397-1410Abstract Full Text Full Text PDF PubMed Scopus (601) Google Scholar use the expression of proteins with well-characterized sizes and mechanical properties to demonstrate that the force-dependent modulation of nuclear pore filtering properties is a general phenomenon that applies to a variety of proteins shuttling between the nucleus and cytoplasm. The direct implication of this phenomenon is that nuclear flattening could affect localization of a large set of proteins, raising the important question of whether this mechanism can induce specific responses. In summary, Elosegui-Artola et al., 2017Elosegui-Artola A. Andreu I. Beedle A.E.M. Lezamiz A. Uroz M. Kosmalska A.J. Oria R. Kechagia J.Z. Rico-Lastres P. Le Roux A.L. et al.Cell. 2017; 171: 1397-1410Abstract Full Text Full Text PDF PubMed Scopus (601) Google Scholar take a major step forward in our understanding of mechanotransduction. They propose a simple model for rigidity sensing, in which mechanosensing through focal adhesions engages actin filaments and the LINC complex to couple substrate rigidity to the nucleus. The resulting nuclear deformation opens up the cytoplasmic side of nuclear pores, which affects the import rate and hence nuclear/cytoplasmic ratios of proteins such as YAP (Figure 1). More generally, this direct effect of nuclear deformation on nuclear pores could also affect transcription and nuclear localization of proteins in other contexts in which forces are exerted on the nucleus, for example during cell migration (Raab et al., 2016Raab M. Gentili M. de Belly H. Thiam H.R. Vargas P. Jimenez A.J. Lautenschlaeger F. Voituriez R. Lennon-Duménil A.M. Manel N. Piel M. Science. 2016; 352: 359-362Crossref PubMed Scopus (519) Google Scholar) or epithelial cell crowding (Saw et al., 2017Saw T.B. Doostmohammadi A. Nier V. Kocgozlu L. Thampi S. Toyama Y. Marcq P. Lim C.T. Yeomans J.M. Ladoux B. Nature. 2017; 544: 212-216Crossref PubMed Scopus (357) Google Scholar). Combined with previous reports showing specific signaling induced by nuclear envelope tension upon osmotic swelling (Enyedi et al., 2016Enyedi B. Jelcic M. Niethammer P. Cell. 2016; 165: 1160-1170Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar) and rupture-repair of the nucleo/cytoplasmic barrier during cell migration (Raab et al., 2016Raab M. Gentili M. de Belly H. Thiam H.R. Vargas P. Jimenez A.J. Lautenschlaeger F. Voituriez R. Lennon-Duménil A.M. Manel N. Piel M. Science. 2016; 352: 359-362Crossref PubMed Scopus (519) Google Scholar, Denais et al., 2016Denais C.M. Gilbert R.M. Isermann P. McGregor A.L. te Lindert M. Weigelin B. Davidson P.M. Friedl P. Wolf K. Lammerding J. Science. 2016; 352: 353-358Crossref PubMed Scopus (698) Google Scholar), this study places the nucleus and its envelope at the center stage of force and shape sensing in single cells. Force Triggers YAP Nuclear Entry by Regulating Transport across Nuclear PoresElosegui-Artola et al.CellOctober 26, 2017In BriefForce-dependent changes in nuclear pores control protein access to the nucleus. Full-Text PDF Open Archive