Fig. 1.TIF

Lamin A/C intermediate filament proteins are constituents of the nuclear lamina contributing to the mechanical stability of the nucleus and can be found throughout the nucleoplasm. Examples of lamin A involvement in transcriptional regulation include laminopathies causing lipodystrophies and progeria. Here, we studied the effect of lamin A KO in murine adult fibroblasts on the organization of chromatin and the DNA binding of peroxisome proliferator‐activated receptor gamma (PPARγ), a transcription factor also implicated in adipogenesis and lipodystrophies. We used confocal microscopy to analyze the distribution of different chromatin marks and fluorescence correlation spectroscopy to measure PPARγ mobility and DNA binding. Conspicuous overall changes were detected in response to both lamin A depletion and treatment with the PPARγ agonist rosiglitazone (RSG). The diameter of the nucleus decreased remarkably in the absence of Lamin A and also upon RSG treatment. Heterochromatin (hc.) was enriched in a distinct peripheral rim, and the thickness of the constitutive hc. rim diminished in the absence of lamin A. The overlap between euchromatin (euc.) and hc. decreased in lamin A KO cells. PPARγ was colocalized with euc. whereas its localization was anti-correlated with constitutive hc., even more so in the absence of lamin A. PPARγ had a fast diffusing fraction transiently bound with short residence times on the DNA and a slow, more stably bound fraction throughout the nucleus including the periphery. RSG increased PPARγ’s colocalization with the euc. and its DNA binding, which was more pronounced in lamin A KO cells. Our results suggest that lamin A plays a primary role in determining nuclear volume and overall chromatin architecture with likely functional consequences exemplified by regulating ligand-induced DNA binding of PPARγ, which may have epigenomic and transcriptional consequences relevant in the context of lipodystrophies.