Nuclear lamins are type V intermediate filament proteins that maintain nuclear structure and function. Furthermore, Emerin - an interactor of Lamin A/C, facilitates crosstalk between the cytoskeleton and the nucleus as it also interacts with actin and Nuclear Myosin 1 (NM1). Here we show that the depletion of Lamin A/C or Emerin, alters the localization of the nuclear motor protein - Nuclear Myosin 1 (NM1) that manifests as an increase in NM1 foci in the nucleus and are rescued to basal levels upon the combined knockdown of Lamin A/C and Emerin. Furthermore, Lamin A/C-Emerin co-depletion destabilizes cytoskeletal organization as it increases actin stress fibers. This further impinges on nuclear organization, as it enhances chromatin mobility more toward the nuclear interior in Lamin A/C-Emerin co-depleted cells. This enhanced chromatin mobility was restored to basal levels either upon inhibition of Nuclear Myosin 1 (NM1) activity or actin depolymerization. In addition, the combined loss of Lamin A/C and Emerin alters the otherwise highly conserved spatial positions of chromosome territories. Furthermore, knockdown of Lamin A/C or Lamin A/C-Emerin combined, deregulates expression levels of a candidate subset of genes. Amongst these genes, both KLK10 (Chr.19, Lamina Associated Domain (LAD+)) and MADH2 (Chr.18, LAD-) were significantly repressed, while BCL2L12 (Chr.19, LAD-) is de-repressed. These genes differentially reposition with respect to the nuclear envelope. Taken together, these studies underscore a remarkable interplay between Lamin A/C and Emerin in modulating cytoskeletal organization of actin and NM1 that impinges on chromatin dynamics and function in the interphase nucleus.

The nuclear pore complex (NPC) mediates nuclear transport of RNA and proteins into and out of the nucleus. Certain nucleoporins have additional functions in chromatin organization and transcription regulation. Nup93 is a scaffold nucleoporin at the nuclear pore complex which is associated with human chromosomes 5, 7 and 16 and with the promoters of the HOXA gene as revealed by ChIP-on-chip studies using tiling microarrays for these chromosomes. However, the functional consequences of the association of Nup93 with HOXA is unknown. Here, we examined the association of Nup93 with the HOXA gene cluster and its consequences on HOXA gene expression in diploid colorectal cancer cells (DLD1). Nup93 showed a specific enrichment ~1 Kb upstream of the transcription start site of each of the HOXA1, HOXA3 and HOXA5 promoters, respectively. Furthermore, the association of Nup93 with HOXA was assisted by its interacting partners Nup188 and Nup205. The depletion of the Nup93 sub-complex significantly upregulated HOXA gene expression levels. However, expression levels of a control gene locus (GLCCI1) on human chromosome 7 were unaffected. Three-dimensional fluorescence in situ hybridization (3D-FISH) analyses revealed that the depletion of the Nup93 sub-complex (but not Nup98) disengages the HOXA gene locus from the nuclear periphery, suggesting a potential role for Nup93 in tethering and repressing the HOXA gene cluster. Consistently, Nup93 knockdown increased active histone marks (H3K9ac), decreased repressive histone marks (H3K27me3) on the HOXA1 promoter and increased transcription elongation marks (H3K36me3) within the HOXA1 gene. Moreover, the combined depletion of Nup93 and CTCF (a known organizer of HOXA gene cluster) but not Nup93 alone, significantly increased GLCCI1 gene expression levels. Taken together, this suggests a novel role for Nup93 and its interactors in repressing the HOXA gene cluster. This study reveals that the nucleoporin Nup93 assisted by its interactors Nup188 and Nup205 mediates the repression of HOXA gene expression.

The nucleolus is required for ribosome biogenesis. Human cells have 2 or 3 nucleoli associated with nucleolar organizer region (NOR)-bearing chromosomes. An increase in number and altered nucleolar morphology define cancer cells. However, the mechanisms that modulate nucleolar morphology and function are unclear. Here we show that in addition to localizing at the nuclear envelope, lamin B2 localizes proximal to nucleolin at the granular component (GC) of the nucleolus and associates with the nucleolar proteins nucleolin and nucleophosmin. Lamin B2 knockdown severely disrupted the nucleolar morphology, which was rescued to intact and discrete nucleoli upon lamin B2 overexpression. Furthermore, two mutually exclusive lamin B2 deletion mutants, ΔHead and ΔSLS, rescued nuclear and nucleolar morphology defects, respectively, induced upon lamin B2 depletion, suggesting independent roles for lamin B2 at the nucleolus and nuclear envelope. Lamin B2 depletion increased nucleolin aggregation in the nucleoplasm, implicating lamin B2 in stabilizing nucleolin within the nucleolus. Lamin B2 knockdown upregulated nucleolus-specific 45S rRNA and upstream intergenic sequence (IGS) transcripts. The IGS transcripts colocalized with aggregates of nucleolin speckles, which were sustained in the nucleoplasm upon lamin B2 depletion. Taken together, these studies uncover a novel role for lamin B2 in modulating the morphology, dynamics, and function of the nucleolus.