Heat-induced morphological and biochemical changes in the nuclear lamina from Ehrlich ascites tumor cells in vivo

Lamin A/C phosphorylation at serine 22 is a conserved heat shock response to regulate nuclear adaptation during stress

The heat shock (HS) response is crucial for cell survival in harmful environments. Nuclear lamin A/C, encoded by the LMNA gene, contributes towards altered gene expression during HS, but the underlying mechanisms are poorly understood. Here, we show that upon HS, lamin A/C was reversibly phosphorylated at serine 22 in concert with HSF1 activation in human cells, mouse cells and Drosophila melanogaster in vivo. Consequently, the phosphorylation facilitated nucleoplasmic localization of lamin A/C and nuclear sphericity in response to HS. Interestingly, lamin A/C knock-out cells showed deformed nuclei after HS and were rescued by ectopic expression of wild-type lamin A, but not by a phosphomimetic (S22D) lamin A mutant. Furthermore, HS triggered concurrent downregulation of lamina-associated protein 2α (Lap2α, encoded by TMPO) in wild-type lamin A/C-expressing cells, but a similar response was perturbed in lamin A/C knock-out cells and in LMNA mutant patient fibroblasts, which showed impaired cell cycle arrest under HS and compromised survival at recovery. Taken together, our results suggest that the altered phosphorylation stoichiometry of lamin A/C provides an evolutionarily conserved mechanism to regulate lamina structure and serve nuclear adaptation and cell survival during HS.

Lamin A/C modulates spatial organization and function of the Hsp70 gene locus via nuclear myosin I

The structure-function relationship of the nucleus is tightly regulated, especially during heat shock. Typically, heat shock activates molecular chaperones that prevent protein misfolding and preserve genome integrity. However, the molecular mechanisms that regulate nuclear structure-function relationships during heat shock remain unclear. Here, we show that lamin A and C (hereafter lamin A/C; both lamin A and C are encoded by LMNA) are required for heat-shock-mediated transcriptional induction of the Hsp70 gene locus (HSPA genes). Interestingly, lamin A/C regulates redistribution of nuclear myosin I (NM1) into the nucleus upon heat shock, and depletion of either lamin A/C or NM1 abrogates heat-shock-induced repositioning of Hsp70 gene locus away from the nuclear envelope. Lamins and NM1 also regulate spatial positioning of the SC35 (also known as SRSF2) speckles - important nuclear landmarks that modulates Hsp70 gene locus expression upon heat shock. This suggests an intricate crosstalk between nuclear lamins, NM1 and SC35 organization in modulating transcriptional responses of the Hsp70 gene locus during heat shock. Taken together, this study unravels a novel role for lamin A/C in the regulation of the spatial dynamics and function of the Hsp70 gene locus upon heat shock, via the nuclear motor protein NM1.This article has an associated First Person interview with the first author of the paper.

Nuclear envelope disorganization in fibroblasts from lipodystrophic patients with heterozygous R482Q/W mutations in the lamin A/C gene

Dunnigan-type familial partial lipodystrophy (FPLD), characterized by an abnormal body fat redistribution with insulin resistance, is caused by missense heterozygous mutations in A-type lamins (lamins A and C). A- and B-type lamins are ubiquitous intermediate filament proteins that polymerize at the inner face of the nuclear envelope. We have analyzed primary cultures of skin fibroblasts from three patients harboring R482Q or R482W mutations. These cells were euploid and able to cycle and divide. A subpopulation of these cells had abnormal blebbing nuclei with A-type lamins forming a peripheral meshwork, which was frequently disorganized. Inner nuclear membrane protein emerin, an A-type lamin-binding protein, strictly colocalized with this abnormal meshwork. Cells from lipodystrophic patients often had other nuclear envelope defects, mainly consisting of nuclear envelope herniations that were deficient in B-type lamins, nuclear pore complexes, lamina-associated protein 2 beta, and chromatin. The mechanical properties of nuclear envelopes were altered, as judged from the extensive deformations observed in nuclei from heat-shocked cells, and from the low stringency of extraction of their components. These structural nuclear alterations were caused by the lamins A/C mutations, as the same changes were introduced in human control fibroblasts by ectopic expression of R482W mutated lamin A.

Quantitative immunofluorescence and immunoelectron microscopy of the topoisomerase II alpha associated with nuclear matrices from wild-type and drug-resistant chinese hamster ovary cell lines

Topo II alpha is considered an important constituent of the nuclear matrix, serving as a fastener of DNA loops to the underlying filamentous scaffolding network. To further define a mechanism of drug resistance to topo II poisons, we studied the quantity of topo II alpha associated with the nuclear matrix in drug-resistant SMR16 and parental cells in the presence and absence of VP-16. Nuclear matrices were prepared from nuclei isolated in EDTA buffer, followed by nuclease digestion with DNase II in the absence of RNase treatment and extraction with 2 M NaCl. Whole-mount spreading of residual structures permits, by means of isoform-specific antibody and colloidal-gold secondary antibodies, an estimate of the amount of topo II alpha in individual nuclear matrices. There are significant variations in topo II alpha amounts between individual nuclear matrices due to the cell cycle distribution. The parental cell line contained eight to ten times more nuclear matrix-associated topo II alpha than the resistant cell line matrices. Nuclear matrix-associated topo II alpha from wild-type and resistant cell lines correlated well with the immunofluorescent staining of the enzyme in nuclei of intact cells. The amount of DNA associated with residual nuclear structures was five times greater in the resistant cell line. This quantity of DNA was not proportional to the quantity of topo II alpha in the same matrix; in fact they were inversely related. In situ whole-mount nuclear matrix preparations were obtained from cells grown on grids and confirmed the results from labeling of isolated residual structures.

Heat-shock and cadmium chloride increase the vimentin mRNA and protein levels in U-937 human promonocytic cells

Heat-shock for 2 hours at 42 degrees C, or the administration for 3 hours of 100 or 150 microM cadmium chloride, inhibited the subsequent proliferation activity, induced the expression of functional differentiation markers, and caused an increase in the amount of the stress-responsive HSP70 protein in U-937 human promonocytic cells. In addition, both heat and cadmium produced an increase in the amount of the intermediate filament protein vimentin, as determined by immunoblot and immunofluorescence assays. By contrast, the amounts of actin and beta-tubulin were not significantly altered. The amount of vimentin mRNA was also increased during recovery from stress, indicating that vimentin expression was not exclusively regulated at the protein level. Although cadmium caused an early, transient stimulation of c-jun and c-fos expression and AP-1 binding activity, heat-shock failed to alter both protooncogene expression and transcription factor binding, indicating that the stress-induced vimentin increase was not the result of AP-1-mediated transcriptional activation. Finally, it was observed that the rate of decay of vimentin mRNA upon actinomycin D administration was decreased in heat- and cadmium-pretreated cells in comparison to untreated cells. These results indicate that stress treatments cause an increase in vimentin levels in promonocytic cells, which may be explained at least in part by transcript stabilization.