Role of nuclear lamins as integrators of extranuclear signals and regulators of genome organization and function

Abstract

The nuclear envelope and its associated proteins integrate different types of intra and extracellular signals perceived by the cell and relay them into the nucleus to elicit signal specific transcriptional responses. The nuclear envelope maintains structure, mechano-responsiveness and plasticity of the nucleus via the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, nuclear Lamins and mechanosensitive proteins such as Emerin that interacts with Lamin A/C. A structurally and functionally pliable nucleus modulates genome organization and function across cellular processes. Studying the role of nuclear envelope proteins as responders and effectors of extranuclear signals is essential to understand the regulation of mechanical and biochemical signal transduction into the nucleus. Here we examined the impact of two different signaling paradigms – i) perception of altered extracellular substrate stiffness and ii) heat shock response, on nuclear structure-function relationships in diploid DLD-1 cells and their regulation by nuclear envelope proteins like Lamins and Emerin. RNA sequencing and 3-Dimensional fluorescence in situ hybridization (3D-FISH) analyses in DLD-1 cells on softer polyacrylamide matrices revealed mislocalization of transcriptionally deregulated chromosomes – Chr. 1 and 19, as well as Chr. 18 with least transcriptional changes, towards the nuclear interior. Furthermore, nuclear Lamins which modulate chromosome positioning, were also mislocalized into the nuclear interior in these cells. We identified a novel phosphorylation of Emerin at Tyr99 in cells on softer matrices, the inhibition of which in a phospho-deficient mutant (emerinY99F), selectively retained chromosome territories as well as Lamins at their conserved nuclear locations. Taken together, Emerin functions as a key mechanosensor, that selectively modulates the spatial organization of chromosome territories in the interphase nucleus in a Lamin dependent manner. Interestingly, we observed that Lamin A and B1 were upregulated during heat shock response in DLD-1 cells. Additionally, Lamins were specifically required for the heat shock mediated induction of the heat shock gene HSPA1A, potentially at two different stages of transcriptional regulation. Remarkably, depletion of Lamin A/C, and not the B-type lamins, abrogated movement of Hsp70 gene locus towards the nuclear interior as well as the nuclear translocation of Hsp70 protein upon thermal stress. Thus, our results highlight a novel role for Lamins in regulating the heat shock response. In summary, this study demonstrates the novel aspects of regulation exerted by nuclear Lamins and their interactors in conjunction, to modulate responses of the nucleus to extranuclear signals.