The A- and B-type nuclear lamin networks: microdomains involved in chromatin organization and transcription

  1. Takeshi Shimi1,6,
  2. Katrin Pfleghaar1,2,6,
  3. Shin-ichiro Kojima1,
  4. Chan-Gi Pack3,4,
  5. Irina Solovei2,
  6. Anne E. Goldman1,
  7. Stephen A. Adam1,
  8. Dale K. Shumaker1,5,
  9. Masataka Kinjo3,
  10. Thomas Cremer2, and
  11. Robert D. Goldman1,7
  1. 1 Department of Cell and Molecular Biology, Northwestern University, Feinberg School of Medicine, Chicago, Illinois 60611, USA;
  2. 2 Institute of Human Genetics, Ludwig-Maximilians University, Planegg-Martinsried 82152, Germany;
  3. 3 Laboratory of Supramolecular Biophysics, Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan;
  4. 4 Cellular Systems Modeling Team, RIKEN Advanced Science Institute, Wako-shi, Saitama 351-0198, Japan;
  5. 5 Department of Urology, Northwestern University, Feinberg School of Medicine, Chicago, Illinois 60611, USA
  1. 6 These authors contributed equally to this work.

Abstract

The nuclear lamins function in the regulation of replication, transcription, and epigenetic modifications of chromatin. However, the mechanisms responsible for these lamin functions are poorly understood. We demonstrate that A- and B-type lamins form separate, but interacting, stable meshworks in the lamina and have different mobilities in the nucleoplasm as determined by fluorescence correlation spectroscopy (FCS). Silencing lamin B1 (LB1) expression dramatically increases the lamina meshwork size and the mobility of nucleoplasmic lamin A (LA). The changes in lamina mesh size are coupled to the formation of LA/C-rich nuclear envelope blebs deficient in LB2. Comparative genomic hybridization (CGH) analyses of microdissected blebs, fluorescence in situ hybridization (FISH), and immunofluorescence localization of modified histones demonstrate that gene-rich euchromatin associates with the LA/C blebs. Enrichment of hyperphosphorylated RNA polymerase II (Pol II) and histone marks for active transcription suggest that blebs are transcriptionally active. However, in vivo labeling of RNA indicates that transcription is decreased, suggesting that the LA/C-rich microenvironment induces promoter proximal stalling of Pol II. We propose that different lamins are organized into separate, but interacting, microdomains and that LB1 is essential for their organization. Our evidence suggests that the organization and regulation of chromatin are influenced by interconnections between these lamin microdomains.

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